The marriage could also have an impact on Harry's friends. They might be caught in the middle between their loyalty to Harry and their respect for Dumbledore. For example, Ron and Hermione might struggle to support Harry wholeheartedly if Dumbledore makes his disapproval clear. Moreover, the witch herself would likely face a great deal of hostility from Dumbledore's supporters, which could make her and Harry's life together quite difficult.
Maybe it's a new character created by the fanfiction author. This witch could have some dark magic tendencies or a past that Dumbledore found unforgivable. For instance, she might have been involved in some unethical magical experiments in the past that Dumbledore strongly opposed.
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.
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.
One consequence could be a split in the friendship group. If Harry hates Ginny, Ron, being Ginny's brother, would be in a difficult position. It could lead to a divide between Harry and Ron as well.
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.
The theme of disillusionment also often appears. Harry, who once idolized Dumbledore, now sees him in a new light. He realizes that Dumbledore is not the perfect mentor he once thought, and this realization leads to his hatred. This can be explored in various ways, such as through flashbacks or interactions with other characters who have different views of Dumbledore.
One possible event could be that Dumbledore fails to protect someone Harry loves dearly. Like if a close friend of Harry's was killed while under Dumbledore's supposed protection, Harry would hold Dumbledore responsible. Another event might be if Dumbledore lies to Harry about his family history. For example, if he hid some important information about Harry's parents that would change Harry's perception of them.
One possible consequence is that Harry could put himself in danger. Dumbledore is usually very wise, and his advice is often meant to protect Harry. By not listening, Harry may walk into a trap or face a more difficult situation than he expected.