Perhaps it's because Harry Potter is used to the real Hermione and the real events in their lives. Fanfiction often adds elements that are not part of their actual experiences. He may feel that these fictional additions take away from the purity of their relationship and the events they've been through. He values the original narrative and the way things actually happened in the wizarding world more than the creative but sometimes inaccurate fan - created stories.
Another possible reason is that Harry is protective of their story. Fanfiction can be quite wild with different interpretations, and he might not like how some fanfictions try to rewrite their relationship. He has a deep - seated understanding of his bond with Hermione, and fanfiction that messes with that can be hard for him to tolerate. He might think that the real story is already great and doesn't need these made - up alterations.
One reason could be that the fanfiction changes her personality traits. In the original, she's kind - hearted but in fanfic, she might be made mean. Harry, who knows the real Molly, won't like this new version.
One reason could be that the fanfic writer portrays Ron as overly jealous of Harry. In the original books, Ron has moments of jealousy but not to an extreme. If in the fanfic it's overdone, Harry might not be able to stand him. For example, if Ron is constantly trying to undermine Harry's achievements out of jealousy.
One reason could be his pure heart. Harry has shown great courage and self - sacrifice throughout his story, which might be seen as 'worthiness' in the context of Mjolnir.
One possible reason could be that he was injured during a battle with dark wizards. Maybe he was hit by a powerful curse that affected his legs. Another reason might be related to some kind of potion gone wrong. For example, if he accidentally drank a potion that had negative side effects on his motor functions.
One reason could be Hermione's excessive worrying. She often frets over small details and Harry might get tired of it and tell her off. For example, if she keeps nagging about the proper way to perform a spell when they are in a hurry during a dangerous situation.
One possible reason could be a miscommunication. Maybe Hermione accidentally reveals a secret of Harry's that he didn't want known, and this causes him to feel betrayed and start to hate her. Another reason could be jealousy. If Hermione gets more attention or praise for her magical abilities than Harry in the fanfic, he might develop feelings of envy and hatred. Or perhaps there's a love triangle situation where Hermione is involved with someone Harry doesn't like, and that causes his hatred.
Fanfiction can sometimes deviate a lot from the original story. It might be that the official creators or those in charge of the Harry Potter brand don't want the characters to be misrepresented in fan - made stories, so they restrict access to it for Harry or anyone in the context of that world. Another aspect is that some fanfiction may have a negative or unethical undertone, and to protect the integrity of the original story, access is prohibited.
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.
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.