Perhaps it's because of a change in Bella's own perception. She might have initially been attracted to Edward, but as time goes on, she realizes that Jasper has qualities that are more in line with what she truly desires. Like, Jasper could be more adventurous or more in tune with her newfound vampire instincts if she has become one, while Edward might be more reserved in certain ways.
It could also be due to some external factors in the fanfiction world. There might be a prophecy or a situation where being with Jasper is seen as more beneficial for Bella or for the vampire community as a whole. For instance, Jasper might have some special powers or connections that could protect Bella better than Edward in a particular upcoming threat.
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 Paul represents a different kind of life for Bella. Maybe he is more down - to - earth compared to the often brooding Edward. Paul might offer a simpler and more straightforward relationship without all the supernatural drama that comes with Edward.
Edward might be initially shocked and heartbroken. He has been in love with Bella for a long time, so her leaving for Jasper would be a huge blow. He could become withdrawn and moody, brooding over what went wrong.
One possible reason could be that Bella discovers a side of Rosalie that she feels more connected to on an emotional level. Maybe Rosalie shows her a different perspective on the vampire world that Edward had been hiding from her. Another aspect could be that Bella has grown tired of Edward's over - protective nature and sees Rosalie as a more independent and free - spirited vampire to be around.
One possibility is that Bella discovers a new side of Jasper that makes her want to leave with him. For example, Jasper might have a hidden ability or knowledge that Bella is intrigued by, and they embark on a journey to explore it further. They could be leaving to find a place where they can be themselves without the constraints of their vampire families or the human - vampire relations in their hometown.
One possible reason could be that he suddenly realizes he's putting Bella in too much danger. In the Twilight universe, Bella's life is constantly at risk because of Edward's vampire world. Maybe in the fanfiction, he decides at the last minute that he can't bear to see her hurt because of him.
One reason could be a shift in the story's setting. If the fanfiction is set in a different time or place, Bella's values and interests might change, leading her to not like Edward. For instance, if it's a modern - day setting with different social norms, she may find Edward's old - fashioned ways off - putting.
Perhaps she has her own vision of their relationship and doesn't want it to be influenced by the often - strange and varying ideas in fanfiction. Fanfiction can be full of outlandish concepts that she doesn't want to be part of. She wants to preserve the unique and special nature of her relationship with Edward as it is in real life.
Another reason might be Edward's overprotectiveness. He could have made a decision for Bella without consulting her first, like preventing her from seeing a friend. This kind of controlling behavior could make Bella feel frustrated and end up in tears.
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.
One possible reason could be that Edward believes Charlie is a threat to Bella in some way that Charlie doesn't realize himself. Maybe Charlie unknowingly gets involved with some dangerous elements, and Edward, with his vampire senses, detects it.