Most people have a phase in their teenage years where they swear up and down they’re “in love” with a person. If you look at Snape’s fucked up home life and the shit he went through at school, it becomes not-so-surprising that he formed an unhealthy attachment to Lily (and really anyone who “accepted” him). James and Sirius’s bullying didn’t help matters at all.
James was allowed to grow and develop a bit. I say “a bit” because it’s said he still tormented Snape in later years, he just did it when Lily wasn’t around. (Can’t let his girlfriend see he’s still being a dick to her former friend.) James also had a healthy relationship with parents who loved and supported him. Hell, everyone loved him.
Snape didn’t have that. Snape is rejected by the girl he formed an unhealthy attachment to, he (most likely) doesn’t have supportive or loving parents, most of the school hates him just based off the fact he’s a Slytherin (even back then Slytherin = evil to most people), and there’s the regular teenage crap. The only other people who are remotely accepting of Snape just so happen to be future Death Eaters. That’s not a coincidence.
I fully believe there was emotional manipulation there too because he was smart as hell and a damn good wizard. They knew they could use him. I do think that without Dumbledore’s influence and with time on his own, he would have seen the truth of his situation. Instead of helping him, Dumbledore saw this hurting, broken, lonely kid and decided “I’m going to use him.”
While we’re on the topic of Dumbledore’s manipulation, think about just how damaging it was. Like, all the people he used spent their lives doing his bidding. He said “jump” and they said “how high”. Their lives were essentially devoted to him and his cause. We saw with Snape he used any means necessary to keep him loyal and in line.
What if Snape, and all of the others Dumbledore manipulated, were allowed to grow and develop? I may be alone on this, but I think without the emotional manipulation and constant reminder of his “lost love” Lily, Snape may have come to realize he didn’t really love her.
I fully believe that if he was allowed to grow up, he would have realized his younger self was just obsessed with Lily. I also believe he would have realized on his own that Voldemort was fucked up and so were the Death Eaters. (Correct me if I’m wrong but, wasn’t he already coming to that conclusion in his teen years? I may need read the books again.)
Dumbledore was a master manipulator and I think the only person to really see him for what he was, was Tom Riddle. Tom saw what he was at that first meeting in the orphanage, and I think Dumbledore knew then he wouldn’t be able to manipulate Tom easily. Tom, as a kid, was able to tease out the truth about horcuxes from a grown ass man. There was no way Dumbledore was controlling him. Tom was a threat. That’s why Dumbledore kept an eye on him for so long. It just so happened he turned into a genocidal villain while Dumbledore was watching.
My point in saying all of this is, a lot of the adults around Dumbledore were emotionally stunted because that’s the way he wanted them. Whether they knew it or not, he was pulling the strings on all of them like an expert puppet master to get the desired outccome. Yes, he was working for the “good side”, but did the means justify the end?