One possible reason could be that Harry has grown tired of being Dumbledore's pawn. He might feel that he has been constantly pushed into dangerous situations for Dumbledore's grand plans and he just wants to focus on his own life and safety for once.
Perhaps Harry has discovered some secrets about Dumbledore that make him question the old wizard's motives. Maybe he found out about Dumbledore's past mistakes or some hidden agendas that make him reluctant to offer his help. For example, if he learned that Dumbledore had a hand in something that put his friends or family in danger, he would be less likely to assist him.
One possible reason could be that Harry might feel he's been used too much in the past by Dumbledore. He could be tired of being thrust into dangerous situations without having a full understanding of the whole plan. For example, in the original series, he often felt like he was just following Dumbledore's orders blindly and it led to great personal risk and the loss of some friends. So in the fanfic, he might have decided enough was enough.
One possible reason could be that he's tired of being used by the British wizarding world. He has faced so many challenges and been through a great deal of trauma, like fighting Voldemort multiple times. He might feel that it's time for others to take responsibility and that he wants to focus on his own life for a change.
One reason could be that Harry has had enough of the wizard world's troubles. He might want a normal life away from magic. After all he's been through, like fighting Voldemort and losing many friends, he may just be tired of the whole wizarding business.
It could also be that Harry Potter in fanfiction is a more self - reflective version of himself. He has already been through so much in the original series that he has grown tired of fighting. He may realize that there are other ways to resolve conflicts, such as through dialogue and understanding, rather than resorting to physical or magical combat as is often depicted in fanfiction.
One possible reason could be a misunderstanding. Maybe Harry overheard something that made him think Dumbledore was planning something against him that he didn't understand. For example, if he heard Dumbledore discussing a dangerous mission for him without his full knowledge.
If Harry Potter refuses to help fanfiction, it might result in some fanfiction losing a bit of authenticity in the eyes of fans. Since Harry is such an iconic character, his absence of support could make some people view certain fanfictions as less legitimate. However, it could also encourage fanfiction writers to develop their own unique voices and storylines without relying on the character's 'approval'.
One possible reason could be that Harry has discovered something about himself or his past that he thinks Dumbledore might not approve of or might try to stop him from exploring further. For example, if he found out some dark magic connection in his family history that he wants to understand on his own without Dumbledore interfering.
It could be that Harry finds out about Dumbledore's secrets regarding the Deathly Hallows. If Harry thinks Dumbledore was more interested in the Hallows for his own purposes rather than for the greater good, it might make him hate Dumbledore.
A possible reason could be jealousy. Maybe Harry sees Dumbledore getting all the praise and respect while he is the one doing all the dangerous work. So, he decides to expose some of Dumbledore's flaws to get some of that attention back.
One possible reason could be that Harry breaks school rules in the fanfiction. For example, if he ventures into the Forbidden Forest without permission. Dumbledore is strict about the safety of students and such disobedience might lead to punishment.