There could be various reasons. Maybe the person Dumbledore wants for Harry doesn't have a good personality or they have different goals and values. Or it could be that Harry simply doesn't feel a connection with that person.
Harry might not marry who Dumbledore wants because of his strong sense of self. He wants to make his own decisions about love and marriage. Also, maybe he sees flaws in the proposed match that Dumbledore overlooks.
Another possibility is that the antagonist is a powerful and ancient magical being that has its own agenda. This being doesn't want Harry to reach his full potential as Dumbledore hopes because it fears that a powerful Harry could disrupt the balance it has been maintaining in the magical world for centuries. So it tries to create obstacles for Dumbledore's efforts to help Harry.
One possible plot could be that Dumbledore wants Harry to take on a dangerous but crucial mission earlier than in the original story. He might be desperate because he sees a dark threat approaching faster and believes only Harry has the unique power to stop it.
The person Dilareba wanted to marry the most was her father. In many documents, it was mentioned that Reba's face was full of happiness when he answered this question, and he said that the person he wanted to marry the most was his father. She believed that her father had no flaws in her heart. He was the perfect father in her dreams. Therefore, it could be concluded that the person Dilreba wanted to marry the most was her father.
The person Wu Song wanted to marry the most was Pan Jinlian. Pan Jinlian was Wu Song's sister-in-law. She had a good impression of Wu Song and had seduced him many times. However, Wu Song never had any feelings for Pan Jinlian. He even scolded her. In the end, Pan Jinlian and Ximen Qing colluded and killed Wu Song's brother, Wu Dalang. Therefore, in order to avenge his brother, Wu Song killed Pan Jinlian. Therefore, although Pan Jinlian had a good impression of Wu Song, Wu Song did not really want to marry her.
In fanfiction, Hermione wanting revenge on Dumbledore could be due to various reasons. One possibility is that she discovers some hidden secrets about his actions that she deems unethical. For example, if she found out that Dumbledore had been using students for some greater, yet morally ambiguous plan. However, this goes against the general perception of Dumbledore in the original series. Fanfiction often takes such unexpected turns to explore different character dynamics and plotlines.
This could be a very dark and interesting fanfiction concept. Maybe Hermione has been deceived by some false information about Dumbledore, thinking he has committed unforgivable acts. Or perhaps she's under some sort of dark magic influence that makes her believe murdering him is the right thing to do. It could also be a plot device to explore the complex character of Hermione and how far she can be pushed from her normal moral compass.
Well, in some fanfictions, Harry might attack Dumbledore if he discovers that Dumbledore has been using him as a pawn in a bigger game all along. This feeling of betrayal could drive him to lash out. Also, if there is a prophecy that he misinterprets to mean Dumbledore is a threat to his future or the future of his friends, that could be a motive.
Snape can be a prominent character. Given his complex relationship with Dumbledore and Harry, his perspective could add a lot to the story. He might have some insights into Dumbledore's actions that Harry doesn't know about, and his reactions to Harry's blaming could vary from being scornful to having a moment of understanding. Luna Lovegood could also be a part of it. Her unique view of the world might make her offer some different viewpoints on the situation between Harry and Dumbledore.
Dumbledore, as Harry's guardian in the fanfiction, would probably use his influence in the wizarding world. He could make sure that Harry is taught by the best teachers at Hogwarts. He'd also be there to offer advice and guidance, especially when Harry faces challenges like dealing with Voldemort or other dark forces. For example, he might teach Harry some advanced defensive magic privately to better protect himself.
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.