One possible reason could be Harry's deep - seated feelings for Ginny. If he has been secretly in love with her, seeing Ginny with Dean might trigger his jealousy and lead to such an extreme reaction like punching Dean.
On the other hand, Ginny might secretly be a bit flattered that Harry is so possessive over her. But still, she would probably try to stop the situation from getting worse and make Harry understand that he can't just go around punching people because of her.
Perhaps in the fanfic, Harry discovers that Dumbledore has been manipulating him all along. Dumbledore has always been a very complex character with many secrets. If Harry were to find out that Dumbledore was using him as a pawn in a much larger and dangerous game, his anger could lead him to punch Dumbledore. This would completely change the dynamic between them and open up a whole new set of storylines and character developments.
In such a fanfiction, Ginny might cheat on Harry with Dean due to a sense of being overshadowed. Harry is often the center of attention in the wizarding world, and Ginny could feel like she's always in his shadow. Dean, on the other hand, might make her feel more special and seen as an individual. Also, it could be that Ginny and Harry have had some unresolved arguments or differences that push Ginny towards Dean.
One possible reason could be that Harry is too focused on his fight against Voldemort and doesn't want to put Ginny in danger. He might think that being with her would make her a target for his enemies.
Well, it depends on how the fanfic is written. Maybe Harry would be heartbroken. He has feelings for Ginny, so seeing her kiss Dean could make him feel like he's lost his chance. He might start to distance himself from Ginny, thinking that she doesn't have the same feelings for him. And this could lead to a whole series of events in the fanfic, like Harry getting more involved in his other adventures or trying to find someone else to distract himself from Ginny.
Well, it could be that in the fanfiction world, Ginny is portrayed as this super desirable character. Harry has a long - standing connection to Ginny, having known her since she was a little girl. But Dean is also attracted to Ginny's independent and feisty nature. So, they end up fighting over her. Maybe there are some misunderstandings too. For example, Dean might think Harry is just being overprotective when in fact Harry has romantic feelings for Ginny, and this leads to a fight.
Well, it could be that Harry is secretly in love with Ginny in the fanfic. He might be too shy to express his feelings directly, so he just stares at her, trying to understand her better or just enjoying her presence.
Maybe in the fanfic, Ginny discovers that Harry has some unresolved trauma from his past, like his experiences with Voldemort. His nightmares and mood swings start to affect their relationship. Harry, on the other hand, might feel that Ginny is too overbearing in trying to 'fix' him. This clash of their approaches to dealing with his past could be a reason for the divorce.
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 possible reason could be that Harry realizes he has stronger emotional and intellectual compatibility with Ginny. He might find that Ginny understands his complex life as a wizard better, especially with her own experiences in the wizarding world. Also, Ginny is more in tune with Harry's adventurous spirit, which Cho might not be as much.