Alec is deeply in love with Magnus and he also just doesn't imagine not being this in love with him, so he's intense (and honest with big actions but not words) but Magnus doesn't really believe it at first, not in the sense that he thinks alec is lying on purpose just that it will pass ("it always pass") or he's accidentally exaggerating ("alec is just like this"). Since Alec thinks it just makes sense that he's this in love with Magnus ("who wouldn't be") he doesn't realise Magnus doesn't fully believe/gets how much he loves him until Magnus is finally present when Alec's about to do something stupid to save/help him or when Catarina sees the way he looks at Magnus and she just sighs and says it's the only time she'll help a shadowhunter and she just says that Magnus isn't aware of much Alec loves him, and he might not be for a little while unless they directly communicate and she leaves
tbh i think that alec is really big and intense with words??? like all the impromptu love declarations and stuff like "when i'm staring into the eyes of the man that i love"... it just sounds so intense, even the verb used (staring instead of like, gazing or something normal) and the general dramatic way alec casually says things (inner strength, which you are overflowing with... seriously what the fuck. i gotta give matt daddario a lot of kudos for managing to pull that one off without sounding cringey or weird, because i remember reading the subtitles for that and thinking "oh this is gonna sound terrible" and then he actually said it and it... didn't. it just sounded like alec is the kind of guy who's intense and dramatic enough to say stuff like this like it's normal. which is kinda the truth but isn't easy to pull off). so honestly i don't even think it's that he doesn't say it enough, it's just... that magnus doesn't really get it, like you said
like he probably has a billion reasons in his head why it isn't really true (why would it be?), some of which don't even make sense together but he's still putting them all together because it just... can't really be real, you know? because again, he's relearning that the default is not that he should be considered a hassle for existing or earning affection in a relationship with someone who supposedly loves him. believing that alec genuinely just... feels for him so strongly, strongly enough to leave earth forever and stay the rest of his life in hell just to be with magnus? that doesn't make fucking SENSE
so in his head i think it's all like... it's his first relationship, of course he's feeling everything so intensely. he's never allowed himself to express love like that before, of course now that he gets to he is so over the top. he's going to get tired of me eventually. i just happened to be the first person he could do this with, but once the novelty dulls he's going to grow tired and find someone he likes more. also, i am somehow too much for him even if he is being way more intense than i am. also, i am probably not intense enough and he will eventually grow tired of that. also, someone else is probably more deserving of all that affection anyway, although at the same time it isn't real and will eventually dull once he calms down. also,
whereas in alec's head it's just like why the FUCK wouldn't magnus know that alec loves him when he says it all the time? and why the fUCK wouldn't alec love him anyway? like it's obviously natural to him, he said that he was going to stay in edom with magnus like it was the most obvious thing on earth because just leaving him there didn't even cross his mind as a possibility. leaving magnus is not a possibility, because alec loves him, and he doesn't turn his back on people just because it's hard
and also i think to alec it's just like... i've said it, so why wouldn't he know? (again autistic icon, like... why the hell do people keep looking for hidden meanings or doubting his words or whatever? he says what he means ffs) like he told magnus time and time again that he loves him and that his past doesn't matter and that magnus is his world and is everything to him so why would magnus ever think otherwise?? it doesn't make sense
also because i think alec never really doubted his feelings. hid them, sure, and doubted whether or not he should act on them, definitely, but he never really doubted his feelings. i think alec is chronically incapable of lying to himself, tbh, he's just too practical and he's very self aware and the fact that he's mostly introverted means he gets a lot of time to contemplate himself lmao. but it was always about his choices - would he say fuck it and be with magnus, or do what's expected of him and marry lydia? - but never about what he felt. he always knew what he felt. again, he's always known he couldn't get what he wanted, which implies he's always known what he wanted. so to him, the way he feels is obvious as all fuck, and there's no reason to doubt it, and it takes him a while to realize that from where magnus is standing, it isn't as obvious
which i think he still doesn't fully get for a long time because again?? he's told him?? and there's probably some angst in there somewhere about alec feeling like he needs to prove to magnus how he feels, but really, it's not about alec at all, it's about magnus and his insecurities and his abuse history, you know? and there's nothing alec can or should do about it
also he's totally heartbroken when he realizes that magnus is still surprised by his affection, surprised to see that alec and his friends came back for him in edom at all, surprised to see alec choose him over and over again. he expects so little and it is so sad to alec because he feels so strongly for him and magnus doesn't get it
at the same time magnus being heartbroken because he knows it's not fair that he keeps doubting alec like that, alec who kept chosing him and surprising him at every turn and never gave him any reason to doubt how he feels, alec who was a lot of magnus' firsts even though magnus is hundreds of years old.... and he probably feels guilty for it too, because alec deserves someonw who understands and trusts him and magnus clearly isn't it. and alec is just more upset by magnus seeing it that way when this is not something magnus controls and alec knows it's got nothing to do with himself. and every time magnus tries to reassure him that it's not his fault, it's not about alec, on the contrary, alec has always been so amazing to him and he doesn't get why it's so hard for him to believe him fully... it just hurts for the both of them
and it's something they slowly have to navigate and learn how to deal with and that i think magnus is getting better and better at, you know, he's healing, and he's been for decades now. but it certainly still hurts for them both to know that it's so hard for magnus to accept love, especially when alec is giving it so freely, and so happy to be able to










