One possible reason could be a desire for a different narrative. Maybe some fans want to explore a darker or more complex relationship. They might think that Dumbledore made some decisions that Harry could potentially resent, like keeping secrets from him.
It could also be about exploring different character arcs. If the fanfiction writer wants to show Harry growing in a different way, having him start off hating Dumbledore could be a starting point. This allows for new storylines, like Harry having to overcome this hatred and learn the truth about Dumbledore's actions in a more roundabout way.
In some cases, it could be a creative take on the characters. Writers might want to show how Harry could feel betrayed by Dumbledore. For example, Dumbledore's use of Harry as a tool to defeat Voldemort without fully explaining everything to him could lead to feelings of hatred in a fictional re - imagining.
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.
One reason could be secrecy. If Harry finds out that Dumbledore has been keeping major secrets from him all along, like about his family or his true role in the wizarding world, he might start to hate him. Another reason could be manipulation. If it seems that Dumbledore has been manipulating Harry's actions for his own gain, that would be a valid cause for hatred in fanfiction.
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 that the 'light' in the fanfiction represents something he associates with strict rules or a false sense of goodness. Maybe it has been used to manipulate people, and Harry, being perceptive, sees through it and thus hates it.
One reason could be that in some fanfictions, Dumbledore's character might be over - idealized or his actions overly scrutinized in ways that make Harry seem secondary. Harry might be tired of always being in Dumbledore's shadow or having Dumbledore's decisions dictating his life even in the fan - created stories.
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.
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.
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.