Well, it could be that Snape framed Harry for something serious, causing him to be expelled and leave Hogwarts. Maybe it was a misunderstanding or a plot by Snape to get rid of Harry for some reason related to their past conflicts.
One possibility is that Snape discovered Harry was involved in a forbidden magic practice and insisted on his leaving. Or perhaps Snape had a personal vendetta against Harry and used his authority to force him out of the school.
In some fanfictions, it might be because Snape exposed a secret of Harry's that led to his departure.
It could also be that Snape represents everything that Harry hates about Hogwarts at that moment in the fanfiction. Snape's strictness, his partiality towards certain students, and his general unfriendly attitude might make Harry feel that Hogwarts is no longer the safe haven it once was. So, he leaves, thinking that he can find a better place where he doesn't have to deal with Snape's negative presence.
One possible reason could be that he feels constantly in danger at Hogwarts. With all the dark magic threats and attacks that keep happening, he might think leaving would keep his friends and himself safer. For example, after the Chamber of Secrets incident, he might have considered leaving to protect others from Voldemort's influence.
One possible reason could be that he feels he has fulfilled his purpose there. Maybe he has achieved certain goals related to protecting Harry Potter secretly, and he decides it's time to move on to other things. For example, in some fanfictions, he might leave to explore his own family history or to further his research on the Dark Arts in a more secluded place.
In fanfiction, one reason could be his desire for a fresh start away from the memories at Hogwarts. Maybe he feels he has done all he can there and wants to explore new magical knowledge elsewhere.
He might leave Hogwarts in fanfic due to a sense of disillusionment. Maybe he discovers some dark secrets about the school or the wizarding world in general that make him question his place there. It could be something related to the way the school is run or a conspiracy among the teachers that he stumbles upon.
Snape's help could have a huge impact on Harry's Hogwarts journey. For example, he might give Harry tips on dealing with certain dark magic threats secretly. Since Snape is a very knowledgeable wizard, his insights into the history of Hogwarts and the secrets of magic could help Harry better understand the school's mysteries and use them to his advantage.
This fanfiction can create a sense of connection among fans. They can discuss how they imagined Snape's memories and what new revelations it brought. It might also inspire fanart or other fan creations based on the ideas in the fanfiction.
Hermione would probably be very shocked at first. She has always seen Snape as a rather cold and unapproachable figure. But after some thought, she might understand Harry's motives if he explained that it was about finding common ground and healing old wounds. Ron, on the other hand, would likely be more openly skeptical and might make some sarcastic comments at first, but eventually come around when he sees the positive impact it has on Harry.
Maybe he leaves because of a conflict with the Hogwarts staff. If the new headmaster or some teachers start implementing rules that Harry believes are wrong or unfair, like restrictions on defensive magic classes, he might decide to leave in protest.
One reason could be to search for a powerful magical artifact outside of Hogwarts that they believe is crucial for defeating a new dark threat. In fanfiction, they might feel that Hogwarts has become too dangerous or restricted in terms of resources for their mission.