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.
One way to make it interesting is by exploring the psychological impact on Harry. How does he cope with this extreme situation? Another aspect could be the reactions of other characters. For example, how do the Death Eaters react to this strange display of power by Voldemort?
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 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.
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.
Maybe some old pure - blood families who want to regain power. They view Harry as a symbol of the new order that they oppose, and Snape as a tool they can use. By forcing this marriage, they hope to control both of them and use their combined influence for their own dark purposes.
In fanfiction, Harry Potter might be angry if his friends are in danger and no one seems to be taking it seriously. He's very protective of his friends, like Ron and Hermione. So if some authority figures in the story are being dismissive of the threats his friends face, that would surely make him angry.
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.
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.
Maybe Harry Potter is tired of the unrealistic portrayals of Snape in fanfiction. Often fanfictions can exaggerate characters' traits, and Harry, who knew Snape in real life, might find these interpretations too far - fetched.
Perhaps there is a political reason. Some powerful pure - blood families might be trying to strengthen their influence by uniting the Potter and Malfoy families through this marriage, even if it's by force. Another possibility is that there is a prophecy that they misinterpreted which led them to force this marriage.