It could also be due to the cumulative effect of all the harm done to him and his loved ones over the years. The wizarding world has been filled with so much conflict and danger, and if he has lost many due to the actions of others, he might reach a point where he just can't forgive. For example, if more of his friends were killed because of someone's negligence or malicious intent, that would harden his heart.
One possible reason could be the extreme betrayal he has faced. Maybe some characters he trusted deeply turned out to be working against him in ways that were unforgivable, like revealing secrets that put his life and the lives of his friends in danger.
It could also be related to the way Konoha has dealt with other issues like the Uchiha clan massacre. If Naruto discovers some aspects of Konoha's involvement in it that he finds morally unacceptable, such as covering up the truth or being complicit in some way. This could make him lose faith in Konoha and be unable to forgive it.
Voldemort is an obvious one. Given all the destruction and death he has caused, it's very likely that Harry would never forgive him. His actions led to the loss of many innocent lives, including Harry's parents.
In the fanfiction, it could be that Harry's aunt and uncle managed to intercept the letter before it got to Harry. They were so determined to keep Harry away from the wizarding world that they might have found a way to stop the owl or trick the postal system. They could have used some kind of Muggle - magic repelling charm they got hold of somehow. This would be a very interesting twist in the story as it shows how far they would go to keep Harry ordinary.
Maybe it's a magical protection. In the wizarding world, there could be some ancient magic associated with hair length. Long hair might act as a sort of shield or a connection to some deeper magic that Harry unknowingly benefits from.
Maybe he's curious about different interpretations of his own story. In fanfictions, his character might be in new and exciting adventures that the original books didn't cover.
Well, in some fanfiction, Harry might annoy everyone because he could be written as overly self - righteous. He's the hero in the original series, but if a writer wants to explore different angles, they might make him constantly spout off about what's right and wrong in a really grating way.
One reason could be a powerful dark magic spell cast upon him by a new villain in the fanfiction world. This spell might be designed to make him disappear from the wizarding world entirely.
One possible reason could be that a powerful dark magic spell has been cast on him. Maybe a new dark wizard or witch, created specifically for the fanfiction, has found a way to make him vanish to cause chaos in the wizarding world. Another reason might be that Harry has chosen to disappear himself. Tired of the constant attention and danger, he decides to go into hiding, perhaps using some long - forgotten magic or a powerful magical object to conceal his presence.
One reason could be a misinformation campaign. Some malicious wizard or witch might have spread false rumors about Harry, making everyone believe that he has done something terrible like being in league with Voldemort again.
In such a fanfiction, it could be that Harry Potter has been corrupted by a powerful and malevolent force. This force might have warped his sense of right and wrong. It could also be that the author wants to explore the extreme consequences of power and how it can turn a hero into a villain. For example, if Harry Potter was constantly taunted and mistreated by the wizarding community despite all he has done, he might reach a breaking point and lash out at everyone.