One reason could be for a strategic mission. Dumbledore might think that as a girl, Harry could get closer to certain people or places that are only accessible to females. Another possibility is that it's part of a magical experiment to understand the nature of magic and gender transformation better.
Maybe Dumbledore wanted to protect Harry from something specific. By changing his gender, he could hide Harry from certain threats. It could also be a way to teach Harry something, like seeing the wizarding world from a different perspective.
Perhaps Dumbledore saw a future danger that could only be averted if Harry was a girl. It could also be related to some ancient prophecy that is linked to a female - form Harry. Or it could be a test of Harry's adaptability and his ability to handle unexpected changes in his identity.
Perhaps there are safety concerns for the students that Dumbledore is not addressing adequately. Harry, having a strong sense of responsibility for his fellow students, may feel that the only way to ensure their safety is to remove Dumbledore from his position. He might believe that Dumbledore's actions or inactions are putting the students at risk, and thus he takes matters into his own hands.
Well, it could be that Harry has a different view on how to handle the war against Voldemort. Dumbledore is often very strategic and sometimes keeps things from Harry for what he believes is the greater good. But Harry might think that full transparency is necessary. Also, Harry may have formed new alliances or friendships that give him different perspectives, and these new ideas conflict with Dumbledore's. He might feel that Dumbledore's approach is too old - fashioned or risky in a new context.
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.
It could be that Dumbledore has been lying to Harry about his own family. For example, if Dumbledore knew more about Harry's connection to the Peverell family and kept it from him. Harry would feel betrayed and might lash out with such a harsh statement.
One reason could be fear. Maybe Harry discovers something about Dumbledore in the fanfiction that makes him scared, like a hidden agenda. Another reason might be to protect Dumbledore. Harry could think that by hiding from him, he is keeping Dumbledore away from some sort of danger that is related to Harry himself. Also, it could be due to a misunderstanding. Harry might misinterpret something Dumbledore does or says, and as a result, decides to hide from him.
It could also be that Harry has his own ideas about how to deal with the situation at hand. He might think that Dumbledore's approach is too cautious or too risky. For instance, if there's a threat from Voldemort, Harry might believe that a more direct and aggressive strategy is needed rather than following Dumbledore's more calculated and long - term plans.
One possible reason could be Dumbledore's over - reliance on his own grand plan. He might have been so focused on the big picture of defeating Voldemort that he overlooked Harry's individual emotional and psychological needs. For example, Harry constantly had to face dangerous situations alone, and Dumbledore didn't always step in to protect him directly. This could have led to Harry feeling abandoned and having to grow up too fast without proper guidance in some aspects.
One reason could be a change in Dumbledore's personality. If he's depicted as being cold or distant in fanfiction, Harry, who knew him as a mentor figure, might not like it.
Perhaps in the fanfic, Harry discovers that Dumbledore has been manipulating him all along. Dumbledore has always been a very complex character with many secrets. If Harry were to find out that Dumbledore was using him as a pawn in a much larger and dangerous game, his anger could lead him to punch Dumbledore. This would completely change the dynamic between them and open up a whole new set of storylines and character developments.
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.