The first season sets Morty up as a well-meaning normal kid possibly with a learning disability with low self esteem and a crush on a girl, who is roped into extremely dangerous adventures both to his physical, mental, and emotional wellbeing. Rick hides his relationship with Morty and once found out manipulates Morty's parents into letting him continue going on adventures without informing them of the various risks.
As the show continues we see them both get closer and more enmeshed – Morty has an opportunity to end the relationship he has with Rick in the last episode of season 1, but is guilted by another adult, Rick's best friend, into saving Rick's ass and keeping him around. Following that decision is a plethora of 'good times' between Rick, Morty, and Summer, which convinces Morty that it was a good choice.
In the following season, the first episode picks off of that and shows us Morty is competing with Summer to be Rick's 'favorite', trying to validate the idea that his relationship with Rick is closer than anyone else's, justified because Rick is the smartest man alive. It feels good for him to like him. Rick shoots this down though, and only makes Morty more committed to pleasing him.
These adventures and intense, moral dilemmas also make Morty feel like he is mentally mature and prepared enough to tackle similar situations - he gets challenged by Rick, either his mindset, worldview, or simply for having boundaries, and once he eventually jumps that hurdle, it is with Rick's aid for the most part. Which reinforces the idea that Morty needs him in order to be good enough for him.
Rick places intense responsibility on Morty for his own emotions and wellbeing, depending on him to take care of him while extremely inebriated, and in dangerous situations. To the point that Morty is familiar with disarming bombs that Rick makes himself while drunk. Rick also makes Morty uncomfortable while drunk multiple times, once because he held a knife to Morty's throat while in a drunken fit of paranoia, and once by insisting on physical affection even though Morty was visibly afraid of him.
Rick consistently forcing Morty to confront his own uncertainty and uncomfortability is eventually seen as a good thing, because Morty is getting outside of his 'normal' shell. Adapting to the environments Rick puts him in makes him a better partner for Rick.
In season 5, it's at the point that Morty cannot live without Rick. There are multiple episodes that address their toxic back and forth, with Rick explicitly stating he has an irrational attachment to Morty and Morty expressing he is desperate to get back together when they 'broke up'. Morty jumps into a sloppy rebound with an older man with bad intentions, and ends up being put in peril because of it. During the event, Morty calls Rick, who was also in a rebound, and becomes more convinced that they need each other because they are the only ones for each other.
Even at the start where Morty first interacted with the citadel, he became shy and pleased with the idea that him and Rick were meant to be together across all realities (until Rick shattered that idea).
Mind you, none of this HAS to be romantic. Rick and Morty are also described as 'best friends', but this in itself is inappropriate because Morty is a 14 year old boy and Rick is his 70 year old grandpa! They have an unhealthy, strong connection that rewards them both, and also negatively affects Morty to the point of severe mental and bodily harm.
The reason that is made okay is because Rick can fix it. He has the power to grow back Morty's limbs when they get cut off, he has the power to bring him back to life when he dies, and he has the power to erase his memories when he becomes too traumatized to function.
It's extremely obvious that Morty has gone through fundamental changes since the start of the show; he is jaded, desensitized, and highly individual. His special ability to get along with Rick as his best friend means he has always been able to tease him, criticize him, and be explosively angry regarding their relationship even if he ends up getting punished for it; the way that they can get over it anyway means in his eyes that it's worth it. He will always forgive Rick and go back to him. Even if Rick is the one that's distant and blows up and blames him, he needs Rick, and he believes Rick needs him.
Even the relationships with girls that Morty does have are either ruined by Rick, or influenced by Rick – The fact that Morty has pursued women much older than himself (Planetina, Stacy, Jacqueline) even is, in itself, proof of Rick's grooming. Morty believes he is mature enough to be with women in their 20s to 30s if he desires them, because Rick has made him mature enough to make these sorts of decisions on his own. The rest of the family, and the supporting cast, and his experiences, just validate that idea.
It's one of the most interesting relationships that I personally have ever seen, and I think rejecting this side of it just because the word 'incest' grosses you out is ridiculous. Because that's what it is. Grooming and complex emotional incest.
