One possible reason could be some sort of political or family alliance within the vampire world. The vampires might have their own complex social structures and rules. If Jasper's family or some powerful vampire faction sees an advantage in binding their families through Bella and Jasper's marriage, they could force it. Another reason might be related to some prophecy or prediction that involves Bella and Jasper together, and the vampires believe that by forcing them to marry, they can fulfill it and gain some sort of power or protection.
Perhaps there is a power struggle within the vampire clans. Some group wants to gain an upper hand by using Bella and Jasper's marriage as a means. Bella, being a human with a special connection to the vampires, is a valuable asset. And Jasper, with his military - like skills and abilities among the vampires, is also important. By forcing them to marry, certain vampires hope to control both of them and use their combined influence for their own political or power - seeking motives. It could also be that there is some misinformation or false belief that Bella and Jasper are 'destined' to be together in a way that requires a forced marriage to come to fruition.
One possible reason could be that Bella discovers a different side of Jasper that she finds more appealing. Maybe Jasper shows a deeper understanding of her in some aspects that Edward lacks. For example, Jasper's past experiences might make him more empathetic towards Bella's own struggles in a way that Edward, with his own set of vampire characteristics, doesn't quite get.
One possible reason could be that Maria sees some potential in Bella that she wants to exploit for her army. Maybe Bella has certain skills or powers that Maria believes can strengthen her military force.
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.
At first, Bella might be shocked and resistant. She could be filled with a sense of disbelief that such a thing is being forced upon her. After all, in the Twilight universe, Bella has her own feelings and desires, especially when it comes to relationships. Forced marriage goes against her free - willed nature. So, she would probably try to find a way to oppose it, perhaps by seeking help from her friends or looking for a loophole in whatever is forcing this marriage upon her.
One reason could be some sort of ancient magical law or prophecy. Maybe there's a hidden clause that says for the greater good of the wizarding world, they need to be married. It could be related to some power or protection that their union might bring.
One reason could be political. Maybe the two clans or villages that Naruto and Sasuke are associated with want to strengthen their alliance. By marrying their most powerful members, they hope to ensure peace and cooperation.
One possible plotline could be that it's a political arrangement. Maybe there's some sort of wizarding world politics where uniting the Potter and Greengrass families is seen as beneficial for stability. Harry might initially resist but then gradually find out that Daphne has her own charm and they start to develop a relationship based on mutual understanding. For example, they could bond over their shared experiences in the wizarding world, like dealing with pure - blood prejudices.
One possible reason could be bullying. In a school or neighborhood setting, stronger or more aggressive kids might force a boy into submission through intimidation and physical threats. For example, they may take his toys or belongings and demand he does what they say to get them back.
One possible reason could be his over - protectiveness. Edward might think that marriage would legally bind Bella to him and he could better protect her from the various dangers in their world, such as other vampires. He may believe that as his wife, he could keep a closer eye on her.
Maybe it's a part of a prank by some mischievous characters at Hogwarts. They might think it's funny to see Harry in a girl's outfit.