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.
Well, it could be due to a magical accident. Perhaps while experimenting with new spells in Hogwarts, something went terribly wrong and it disabled his ability to walk. Or it could be that he was cursed by a vengeful enemy who specifically targeted his mobility. In some fanfictions, there could also be a plot where he sacrifices his ability to walk to save his friends from a great danger.
Maybe he got injured during a dangerous adventure. For example, he could have fallen from a great height while chasing a dark wizard and broken his legs.
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.
In the 'Harry Potter can't walk fanfiction' when Harry loses his ability to walk, the story could develop in a very emotional way. His friends and allies would be determined to find a way to restore his ability to walk. Meanwhile, Harry himself might start to see the wizarding world from a different perspective. He may notice things that he overlooked before when he was able - bodied. He could also become a symbol of perseverance for other students who are facing difficulties. The story could also involve Harry learning to use magic in new ways to compensate for his lack of mobility, such as using telekinesis spells more effectively.
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 reason could be a reaction to the Ministry of Magic's mismanagement. In the original story, the Ministry often made mistakes and had corrupt elements. In fanfiction, this could lead to Harry becoming an anarchist as a way to oppose such a flawed system.
One possible reason could be that the magic used against him is of a very dark and ancient nature that defies the normal healing spells. For example, if it's a curse created by a long - forgotten dark wizard with unique and unbreakable properties.
Harry might walk away because he has a new mission that he can't share with others. He realizes that to protect the wizarding world in a more fundamental way, he has to go it alone. So, he quietly walks away from his normal life to pursue this unknown and dangerous task.
One reason could be to add more depth to his character. Schizophrenia can bring out complex emotions and behaviors, making Harry more relatable in some ways.