In a fanfiction, Dumbledore might steal out of desperation. Suppose there was a situation where a curse was about to be unleashed on Hogwarts, and the only way to stop it was by stealing a particular ingredient for an antidote from a place where it was not supposed to be taken. Dumbledore, seeing no other option, decides to steal it. His motive here is to save the school and its students, even if it means breaking the rules.
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.
One possible motive could be a misguided sense of protection. Dumbledore might think that taking something from Harry, like a powerful magical item, would keep it out of the wrong hands. He may believe that Harry is too young or inexperienced to handle it properly.
Perhaps Harry has been misled by someone he trusts. They could have filled his head with lies about Dumbledore, making Harry think that Dumbledore is actually a threat. And his motive would then be to protect the wizarding world from what he believes is a hidden danger in Dumbledore.
One possible motive could be protection. Dumbledore might tie Harry to the bed to keep him from running off into danger. He has always been protective of Harry and perhaps in this fanfiction's unique scenario, he believes this extreme measure is necessary to safeguard Harry from some immediate and unseen threat.
I'm not sure if there is a specific 'Harry Potter Dumbledore Stealing Fanfiction' out there. However, if there were, the plot could potentially involve Dumbledore stealing something important like a powerful magical artifact that he believes is necessary to protect Hogwarts or the wizarding world from a great evil, but his actions are misinterpreted by the characters in the fanfiction.
One possible reason could be that Dumbledore believes he is acting in Harry's greater good. Maybe he thinks that by taking certain items from Harry, he can protect them from falling into the wrong hands. For example, if Harry has a powerful magical artifact that Voldemort desires, Dumbledore might steal it to keep it safe.
One possible motive could be to explore a dark and alternate universe. The writer might want to see how the 'Harry Potter' world would change if such a drastic event occurred. Another reason could be to analyze the characters from a different perspective. By making Harry the destroyer of Dumbledore, it forces readers to re - evaluate both characters.
It could be that Dumbledore is afraid of what Harry might do if left to his own devices. Harry has a lot of power, and Dumbledore may worry that he could be swayed by the wrong people or ideas. So, in an attempt to keep Harry on the 'right path', he resorts to trying to control him. But this also shows a lack of faith in Harry's character and judgment.
Some writers might do it to create a sense of shock. They want to break the traditional view of the Dumbledore - Harry relationship and get readers' attention. It's a way to be different in the vast sea of fanfiction.
Maybe Dumbledore believes that Harry has some hidden power that is being corrupted by the environment at Hogwarts. So, he kidnaps Harry to a secluded place to train him and help him control this power without any distractions or negative influences from other students or even some of the teachers.