One possible reason could be that she is in a desperate situation. Maybe she needs something for a very important spell or potion to save someone's life and she thinks Harry won't notice or will forgive her. For example, if there is a rare ingredient that only Harry has access to and time is running out.
Perhaps she is being influenced by some dark magic or a cursed object. This could cloud her judgment and make her do things she wouldn't normally do, like stealing from her friend. It might be that the object she steals has some sort of power over her that she can't resist.
Another reason could be a misunderstanding. Maybe she believes that something Harry has actually belongs to her or was wrongly taken from her in the first place. So in her mind, she's not really stealing but reclaiming what is rightfully hers.
One possible motive could be that Hermione believes the grimoire contains powerful magic that she thinks is necessary to defeat a great evil. She might think Harry is not using it to its full potential or is unaware of its true importance.
Maybe she's desperate for some knowledge that she thinks Harry has in those books. For example, if the books are related to some dark magic or ancient spells that she wants to study for the greater good, she might be tempted to take them without asking.
In some fanfictions, there might be a new magical element or prophecy that makes Harry question Hermione's motives. If the prophecy says something about Hermione being involved in a dark plot, even if it's false, Harry may start to have doubts about her.
It could also be due to a series of unlucky events. Suppose Hermione's well - intentioned plans keep backfiring in a way that makes Harry question her judgment. Like when she tries to solve a problem but ends up making it worse, Harry might start to lose trust.
Another reason could be that there are some misunderstandings between them. For example, Hermione might misinterpret Harry's actions or words as being too reckless or irresponsible, and she decides to push him away to avoid being dragged into what she perceives as a bad situation. Also, it could be related to her own insecurities. She might feel that she is holding Harry back in some way or that their friendship is becoming too complicated, so she takes the step of pushing him away to simplify things.
One reason could be jealousy. Maybe Hermione is getting more attention in school for her academic achievements, and Harry, feeling overshadowed, acts mean. For example, in some fanfictions, he might make snide remarks about her always having her nose in a book.
One possible reason could be a misinterpretation of their relationships. Maybe the author wants to explore a different dynamic where Harry and Hermione bond over Ron's perceived flaws, like his occasional laziness or his insecurities that might come across as annoying to them in this fictional scenario.
Jealousy can also be a factor. Maybe Harry becomes jealous of Hermione's close friendship with another character, or vice versa. This could create tension and ultimately result in a break - up. In some fanfictions, Hermione's intelligence and success might overshadow Harry at times, causing him to feel insecure in the relationship.
Power could be a reason. If there's a chance for Ron and Hermione to gain a great deal of power by turning against Harry, some fanfictions might explore this. For instance, a new dark power might offer them something they've always desired, like the ability to reform the wizarding world in their own image. They might think Harry would oppose this, so they betray him to get what they want.
It's possible that Harry hates it because he has a certain image of Ron and Hermione in his mind. Fanfiction can sometimes create versions of them that are completely different from what he knows. For example, some fanfiction might focus on a love triangle that he doesn't think exists in their real relationship. This divergence from his perception could lead to his dislike.