In the fanfiction world, it could be a prophecy. There might be a prophecy that says Harry's presence in Ginny's life will bring great harm to her or the wizarding world. So, for the greater good, Harry leaves. He believes that by sacrificing his relationship with Ginny, he can save her and everyone else. It's a very noble but also a very heart - breaking decision.
Another reason could be a misunderstanding. Maybe he overhears something Harry and Hermione say and misinterprets it as them not wanting him around anymore. So, in a fit of anger and hurt, he leaves. This can lead to a lot of angst and drama in the fanfiction as the other characters then have to figure out how to get him back.
She might hate it because it could change the way people see her character. In the original story, she is a strong and independent person, but fanfiction might portray her in a wrong way.
Maybe in fanfiction, Ginny discovers that Harry still has some unresolved feelings for Hermione. This could create a rift between them and lead to a divorce. It's not uncommon in fan - created stories to explore complex love triangles that were not fully resolved in the original series.
It could be due to jealousy. Perhaps Ginny has been spending a lot of time with another character, and Harry misinterprets their relationship, leading to his anger. In fanfiction, the writers often play with such emotions to create drama.
One possible reason could be a misunderstanding. Maybe Ginny misinterprets something Harry does and feels she can't stay in the relationship. Another reason might be that she feels overshadowed by Harry's fame and the pressure of being part of the Potter family. It could also be that she has some personal goals or dreams that she realizes she can't pursue while being with Harry and taking care of the kids.
One possible reason could be jealousy. Maybe Ginny feels overshadowed by Harry's fame even within their relationship and she strays to someone who makes her feel more special on a personal level. Another reason could be miscommunication. If they have been too busy with their respective wizarding duties and haven't been communicating well, Ginny might feel neglected and be drawn to someone else who seems to give her more attention.
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
One reason could be a falling - out with his friends in Gryffindor. Maybe there was a misunderstanding that grew so large that he felt he couldn't stay. Another possibility is that he might be lured away by some mysterious force or character who convinces him that Gryffindor is no longer the right place for him. For example, if a new group claims to have a better way to fight Voldemort and they are from a different 'house' in this fanfiction concept.
In some fanfictions, Ginny may reject Harry because she wants to focus on her own personal growth. She might feel that a relationship with Harry at that time would hold her back from achieving her own goals, like becoming a great Quidditch player or an accomplished witch in her own right. So, she decides to reject him to pursue her own dreams first.
One reason could be that the fanfiction is set in a different universe where the characters' personalities are slightly altered. So, this 'new' Harry just doesn't click with Ginny in that version. For instance, if the fanfic is a darker take on the story, Harry might be more jaded and not open to a relationship with Ginny.