Maybe it's a part of a prophecy that no one fully understands. The magic of the prophecy forces Harry to be with Snape in this way. It could also be that Snape himself has been cursed, and as a result, Harry is drawn to him in this forced 'ride' situation because the curse has some sort of connection to Harry. Or perhaps a magical object has been activated that binds them together for a specific purpose that has yet to be revealed.
In fanfiction, it could be that the Ministry of Magic has some devious plan. They want to keep an eye on both Harry and Snape, and by forcing them together in this strange 'ride' situation, they can monitor their interactions more closely. Or it could be related to a power struggle within Hogwarts. There are factions that believe that by making Harry and Snape interact in this forced way, they can tip the balance of power in their favor, whether it's for good or for evil purposes.
One reason could be a misfired spell by another character. For example, if a young wizard was experimenting with a transportation spell and it backfired, it could accidentally force Harry to be attached to Snape in this 'ride' situation. Another reason might be a plot by a dark wizard who wants to see chaos at Hogwarts. They figure that forcing Harry and Snape together in this strange way would disrupt the normal order of things.
Well, it's a really strange and dark concept. If we consider this in a non - inappropriate way, perhaps it could be a situation where Harry is magically compelled to follow Snape on a journey, like on a broomstick ride. Maybe Snape has a mission and needs Harry's presence for some unknown reason, and he uses magic to force Harry to come along with him. It would be a very tense situation, with Harry being angry and Snape being his usual brooding self.
One reason could be that Harry sees Snape's Patronus, which is the same as his mother's. This could be a strong indication of Snape's true feelings and make Harry trust him.
Perhaps in this fanfiction, Snape shows a side of himself that is completely different from what Harry has known. He might be more open and honest with Harry, sharing secrets that make Harry believe he can be trusted. Like Snape might disclose some long - held information about Harry's parents that only he knew, which makes Harry think Snape has been on his side all along.
In such a fanfiction, it could be a very strange and dark scenario. Maybe Voldemort is using some sort of dark magic to control Harry and force him into this demeaning situation. It could be part of a plot to break Harry's spirit or use him as a tool in some dark ritual.
One possible reason could be Snape's lingering resentment towards James Potter, Harry's father. Snape might see Harry as a reminder of the bullying he endured from James during their school days. So, he takes out his anger on Harry.
One reason could be that in this fanfiction, Snape has some hidden kindness towards Harry that he doesn't show in the original story. Maybe he secretly protects Harry from other threats, which makes Harry feel safe around him.
Harry would probably be quite resentful at first. He might think Snape is just being his usual unfair self. So, he would be sulky and do the tasks reluctantly.
Perhaps he likes to see how fans re - imagine his character. He might be curious about how others view his actions and motives throughout the story.
One reason could be that the creators of such fanfiction are die - hard Snape fans. They might feel that Snape is the most interesting and underappreciated character in the 'Harry Potter' series. So, they create fanfiction where Harry Potter, who they may see as getting too much attention in the original, is not as great, to put more focus on Snape.
One possible reason could be copyright infringement. If the fanfiction uses too much of the original 'Harry Potter' material without proper authorization, J.K. Rowling (or those in charge of the franchise) might take legal action. Another reason could be if the fanfiction misrepresents the characters in a way that goes against the intended spirit of the original work.