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Their relationship deteriorated because it was full of miscommunication. They just had these expectations and views of each other that were never fulfilled, I suppose. They couldn’t understand each other, and that was the main problem. Snape probably idealized Lily, while Lily didn’t understand Snape, so she couldn’t see why he kept hurting her in her eyes. They loved the idea of having a best friend, but I don’t think they ever truly knew each other. What kept Lily coming back was nostalgia, safety, and probably shared interests.
I absolutely hate the whole 'snape loved lily' plot twist. People often equate it to the way Sirius still loved and missed james years after his death and went to insane lengths to protect his son, but tht was so not the case with snily.
Coz snape and lily had such a toxic, imperfect relationship in canon, but instead of treating it tht way it was never addressed. It wud hv been so interesting to explore this, but the series just doesn't.
Both of them r awful friends to each other. Snape's faults r obviously pretty clear (aiming to join a terrorist group tht believes ppl lyk his friend shouldn't be alive...yea tht doesn't need to be mentioned twice lol). Don't get me wrong, lily was completely right to cut Snape off, but it wasn't lyk she was a good friend even before tht. Can u ever imagine Ron or Hermione almost smiling if Draco was exposing Harry's underpants to a crowd of jeering students? Nope. They wud throw hands, period. Can u imagine any of the marauders doing tht to each other? (their internal dynamics might not be the healthiest but they wudnt stand for it if an outsider bullied their friends). Absolutely not. Coz it's not about these characters, it's just... not how u react when ur 'friend' is getting tormented (or anyone else for tht matter, but friend in this context specifically). Also can u imagine Ron or Hermione relying on school rumours saying tht Draco saved Harry's life without asking Harry about his version of the event first? Nope. Can u see what I'm getting at here?
Lily wasn't even a good friend to Snape to begin with, but he loved her until the end (irrespective of whether u want to see it as romantic or platonic). Now, I cud buy the narrative tht lily was the only person who was ever kind to him and thts y he clung on to her, but thts not how canon sees it. The source material doesn't address the nuances of this relationship in any meaningful way other than 'snape loved lily and tht is y he switched sides'.
Which is y the reason he switches sides, and his continued love for lily until he dies at 38 seems sort of shallow to me. Coz it wud be different if he'd hurt someone who truly loved him, but thts clearly not the case (u cud say tht lily loved him and did wrong by him too but tht isn't canon, given tht canon doesn't think she ever wronged him to begin with). Just feels lyk he was gaslit by the author tbh