I think not enough people understand that Astarion doesn't want you to be his main or only source of blood. This even stands in direct contrast to what he really wants to achieve with biting Tav. Let me explain.
Why Astarion doesn't want you to be his blood bag
Let me start with explaining how I approached this topic. First of all, when I triggered the bite scene in my playthrough, I obviously let Astarion drink from my Tav. But then I got curious. What happens when I don't stop him? He's said I could trust him just a moment ago, didn't he? So I did just that, I trusted him literally with my life.
Aaaand... he killed me. Well. What did I expect from a vampire, really. But that he actually drained me dry broke my heart. I really wanted to trust him, to reach out and show him that I didn't judge him for what he was and so on. I was really disappointed in him and couldn't quite grasp why he would do this. Was it a conscious decision? Did he lose control? Surely he would apologise and explain everything to me once someone resurrected me, returning the trust I was giving him the night prior, right?... right?
Of course not. The dialogue after him killing Tav was... at least as disappointing as him killing Tav in the first place. He shows his usual attitude, apologises half-heartedly and then just keeps going on with talking about draining the occasional bandit. He even snaps when you mention the topic of him feeding after that with something like "I already apologised, what more do you want?".
There's no real regret, no emotions. He simply doesn't care.
We know that he didn't care in the beginning, he tells us as much when he confesses his unwanted, growing feelings towards Tav in act 2. But still the whole bite scene didn't sit right with me until…
The Nightmare
I randomly stumbled upon a clip of Astarion having a nightmare, so I researched about it and even started an Astarion run to experience it myself. So, if you play as Astarion, at the second long rest of act 1 he has a nightmare about Cazador, in which he recites the rules that defined how Astarion and the other spawn had to live:
First, thou shalt not drink of the blood of thinking creatures.
Second, thou shalt obey me in all things.
Third, thou shalt not leave my side unless directed.
Fourth, thou shalt know that thou art mine.
However Astarion reacts to this dream, he jolts awake afterwards and instinctively panics that he needs to find a way back to his master as soon as possible. His eyes set on his companions, who are sleeping / meditating peacefully around the fire, and then a thought passes his mind.
He could try to break one of Cazador's rules right then and there. He's able to stand in the sun, to bathe in running water, so maybe…
And that's why he decides to bite a companion. Astarion wakes up utterly terrified of what his master will do to him if he doesn't return to him in an instant - and he knows all too well what kind of horrors would await him if he so much as dares to think about rebelling again (read about that here) - so he is in desperate need of confirmation that he is now able to withstand and actively break the rules that have dominated his life for two centuries.
He needs to know that he is able to drink the blood of a thinking creature. And there they are. His clueless companions served on a silver platter. It's like an invitation to him, to test his boundaries. And who would be more fitting than the good hearted leader of the party Astarion wanted to (or already has) seduce(d) anyway?
This piece of information shed a whole new light on the bite scene. But let's look a bit closer at that.
The Bite Night
The very first thing Tav registers about what's going on with Astarion that night is him baring his fangs right above us, about to sink them into our flesh. He pulls back as soon as we open our eyes, retreating immediately until there's a safe distance between him and Tav.
This may be the first time Tav gets to know that Astarion is a vampire, so he gets defensive and tells us that he's never killed a person for food, only animals. But then, instead of letting the idea of feeding on Tav go, he insists that animals aren't enough.
But it's not enough. Not if I have to fight. I feel so weak.
If I just had a little blood, I could think clearer. Fight better. Please.
Then he goes on, literally pleading to get what he wants while putting on a sad expression in the end, even averting his gaze.
And if we now have in mind that he just woke up from a nightmare about Cazador, feeling powerless and in desperate need of even a glimpse of hope that he might escape Cazador's cruel grip, his lines make a lot of sense. He's veiling the truth, of course, but not all of what he says is a lie.
He indeed feels weak - powerless in fact - so drinking Tav's blood (the blood of a thinking creature) could prove that he may have regained a bit of power over himself, which had been exclusively reserved for Cazador the last 200 years. This would absolutely make him feel stronger, more confident than he's felt for a long, long time.
And he indeed could think clearer after tasting Tav's blood, because this would bring clarity to the question if he is now able to break Cazador's rules on purpose and therefore give him information to work with when he needs to think about what to do next.
Right after this, we get the chance to push into Astarion's mind, and if we do this, we can see what he thinks about.
His mind opens up, revealing cracked and quivering memories. At their heart, you see dark eyes, commanding you to feed.
You open your mouth and bite down. Not into a tender neck, but in the twisting body of a rat - the only thing your master lets you eat.
He recalls the memory of how his master used to force him to eat disgusting vermin. As his memory is told, we can clearly see how deeply this affects him, for he can't keep up his walls of defensiveness and detachment. They just crack, crumble and reveal his misery clearly for Tav to observe. When we ask him about it directly afterwards, he first seems to search for a way out - a witty remark, anything - but gives up almost immediately. And he looks and sounds just completely defeated and tired, confirming what Tav just saw in his mind.
He goes on with talking about trust. I think here he's seeing a chance to gain Tav's sympathy. If he already reveals such delicate information about his past, he can as well make use of it. So he appeals to Tav's understanding, offering us that this past is the reason for him only trusting Tav slowly. But then he immediately adds that now he trusts Tav, and that in return Tav can trust him, too.
What he does here is displaying himself as pitiful, gaining Tav's sympathy, then seemingly going out of his way and saying that despite all he does trust Tav, which puts Tav in the position to follow suit with returning the trust… which Astarion definitely lied about on his part. But that's what he does. Manipulating.
If we then respond with "You tried to bite me. How can I trust you?" instead of assuring us of his trustworthiness with more manipulation, he gets frustrated.
Because we don't have a choice! Not if we're going to save ourselves from these worms.
But he goes on with masking his reasons, even if he gave us a hint right in the beginning of act 1 when we picked him up, where he said that he would rather control the tadpole instead of removing it. If we play as Astarion, it gets clear pretty fast that he holds onto the tadpole, because it seems to be the only thing that had been able to "save" him from Cazador whilst no one and nothing else even attempted to help him for two centuries. Of course he would not want to get rid of the tadpole just like that. But he says so nonetheless to align with Tav's goals and display himself as useful.
And then he does something interesting.
I need you alive. You need me strong.
Please. Only be a taste, I swear. I'll be well, you'll be fine, and everything can go back to normal.
In the beginning of this conversation he mentioned he needed the blood to fight better. Now he takes up on this by saying that Tav needs him strong, hinting on him getting stronger after having a taste of Tav's blood. And then he promises that after this "everything can go back to normal", which right now means Astarion feeding on animals again.
A few things about this are odd.
Firstly, "only be a taste". Everyone, literally everyone has heard about the insatiable hunger of vampires. So how is it supposed to work that Astarion gets stronger from just a taste of Tav's blood? For how long? A few hours? Not nearly long enough to pose a real advantage, eh? For me this makes no sense. And if we think about his true intentions - wanting to find out if he can break Cazador's rule - just a taste would be absolutely enough. (And after he bites Tav without killing them, he even says that he needs something more filling!)
Secondly, if Astarion really wanted to become stronger with the help of Tav's blood, why would he promise to go back to normal afterwards? He just offered Tav a stronger companion but then immediately nullified this argument by literally saying that this will be a one time arrangement.
Thirdly, he subtly offers a bargain. "You give me your blood, I will be a stronger fighter for you." He did so in the beginning as well, repeating it with different words. And it fits his character very well to do so, because for all he knows everything comes with a price. He almost gets beaten to death and Cazador mercifully comes to his rescue? The price is a never ending life of torment and abuse. Astarion helps a potential victim for Cazador to flee? The price is a year of starvation, locked up in a dusty and dark tomb without knowing if it will ever find an end. Mindflayers rescued him from Cazador, (passively) granting him to possibly be free of him at last? The price is becoming a tentacled monster in the end.
If we then allow him to bite us, he's visibly surprised about our graciousness, but of course doesn't let this chance slip and suggests getting comfortable instantly. Then he finally gets to sink his teeth into Tav's neck. This part of the scene can more or less be viewed from both sides - Astarion's and Tav's.
He begins to feed on Tav and after a bit we can decide to interrupt him, but have to pass an ability check first. This repeats a second time when playing Tav, and even if it's only one AC when we play Astarion and decide to bite a companion, it still aligns, showing that Astarion seemingly loses himself in the taste of Tav's blood - which is very likely because (as he later tells us) we were the first humanoid he's ever fed on, so it's imaginable that Tav's blood must be tasting almost divine to him.
This theory is supported by his actions after we fail the first AC or just let him continue. He grabs the back of Tav's head to pull them towards himself, emphasising on how greedy he is sucking the blood out of Tav. The camera even uses the exact same angles before failing or skipping the first AC and after, so the comparison is easy and the difference is clear:
If we then fail or skip the next AC, Astarion just drains us dry… And it gets even better after the bite night.
The Morning After
When Tav gets revived and then talks to Astarion, he is visibly surprised to see us, after he left the dead body of Tav behind with saying "Oh no, something terrible has happened". Sure Astarion, something…
All of this happens when Astarion still doesn't care for Tav. He reacts with panic when we confront him with the fact that he literally killed us, and just manages to get his expression under control after a few seconds. I mean, just look at him:
'Killed' feels like a strong word. Not many corpses have your vigour.
He definitely knows what he's done, and that 'killed' is indeed the right word for it, he is just very very bad at coping with guilt (which is amazingly analysed by thelikesoffinn - definitely read this, you'll understand Astarion so much better after this).
He instantly lays the focus on our codependency again, that we need each other and so on. If we question why we should keep him around, he answers with:
A strong, well-fed vampire? I'm a powerful weapon - you'd be a fool to toss me aside now.
With mentioning this, he wants to make sure that Tav will let him stay by their side and therefore grant him protection.
But more importantly he continues:
Anyway, last night was an aberration. It will never happen again.
He doesn't even start with something like "Next time I'll be more careful" or anything similar. He straight up says that it will never happen again. Period. If we then ask who he will feed on next time he gets hungry, he presents the idea of feeding on villains and bandits "who need killing anyway". And this is exactly what he wants. This even shows in his reaction to Tav's response to his suggestion.
If we agree and therefore allow him to feed on our enemies ("Sounds good. Glad we could agree"), this is how he reacts:
As am I. I'm starting to feel a little peckish already.
This itself doesn't sound all too enthusiastic, but we get his approval up, which definitely shows that he very much likes that Tav agreed.
On the contrary, if we suggest he can feed on Tav, ignoring his will wish to feed on villains ("Look, I'm not against you feeding on me, but only if we talk about it first"), this is his reaction:
Of course! This sounds eminently reasonable.
I shall wait patiently until you suggest we… dine together.
Doesn't sound too bad either, eh? But we don't get his approval. He doesn't disapprove, of course, because this is still a thousand times better than what he had until then, but still not what he really wants…
So, what does all of this mean for the initial question?
Conclusion
The crucial point here is what it means for Astarion to feed on Tav. The only things he remembers since Cazador turned him, are being relentlessly dominated and horrifyingly abused. The things Astarion wants the most are to be safe and to finally have control over himself again.
He bit Tav for the sole purpose of finding out if he can be free of Cazador's rules. So why would he jump right into being dependent on Tav? He suggests to feed on villains, because then he is free of anyone's mercy. He doesn't need to rely on Tav to graciously allow him to get a drop of blood. He can do this himself now.
This is such an important step for his character growth, to find the way to his autonomy again, so if we only allow him to feed on Tav, it instantly sets him back into old habits of bowing to his masters words - or in this case Tav's. Because it's all hes been doing for the last two centuries of his life.
So, as much as the thought of the self-sacrificing offer to be his personal blood bag may seem romantic or whatever, it's actually the exact opposite, trapping Astarion in what he desperately tries to escape from. The restrictions that come with someone dominating him mentally and physically. And as I mentioned earlier, he doesn't believe in the goodness of people. For him every "kind" act has a price and he likes to know what he has to pay, so he wouldn't even want to just drink Tav's blood without Tav getting anything out of it. He would most certainly expect Tav one day to come around with something he doesn't want to give or do, so he wants to control such situations beforehand.
All he wants is to make his own decisions and be free in every way possible…
So please just let him drain some bandits, will ya?
