It could be that some external events in the fanfiction have caused a rift. For example, perhaps Trunks did something that Vegeta disagreed with strongly. It might be related to Trunks' choice of a different path in training or his association with certain characters that Vegeta doesn't like. In fanfiction, the writers often create these kinds of conflicts to explore different aspects of the characters.
Maybe in the fanfiction, Vegeta has some personal growth that makes him focus more on himself rather than family. He might have new goals and ambitions that he thinks are more important than his relationship with Trunks.
Gohan might try to be a better friend to Trunks. With Vegeta's change, Gohan, who has always been a good - hearted character, may step in to support Trunks. He could offer his own training methods or just be there as an emotional support. This would strengthen their friendship and might also create an interesting dynamic between Gohan and Vegeta, as Vegeta might see this as an intrusion or a sign of weakness on Trunks' part.
Perhaps Sakura has grown and changed herself. She might have developed new interests and values that are no longer compatible with Sasuke's. In the fanfiction, she could have met new people who showed her a different kind of love and respect, making her question her feelings for Sasuke.
It could be that the character has found someone else who shows more care and attention. In fanfics, new characters can be introduced who are more in tune with the main character's emotional needs. Unlike Vegeta, who may be seen as too cold or unresponsive.
Another plot could be that the character who says this has met someone else who shows them a different kind of love and attention. They start comparing this new person to Vegeta and realize that they are no longer in love with Vegeta. This new relationship makes them see all the flaws in their relationship with Vegeta that they were previously overlooking.
One possible storyline could be that Naruto has faced so many disappointments that he decides to isolate himself from the Hidden Leaf Village. Maybe he feels betrayed by the people he once trusted, like some of the village elders. He might then embark on a journey alone, exploring new lands and meeting different shinobi cultures.
It could be that Sakura has done something that has disappointed Naruto deeply. This could range from her making a decision in a mission that Naruto didn't agree with, to her not being there for him during a difficult time. These actions might have gradually eroded his love for her.
It could be that Vegeta is influenced by some external force or an old Saiyan belief that makes him act in a way that hurts Trunks. Maybe he believes that only through extreme hardship can Trunks reach his true potential. So he subjects Trunks to dangerous situations or withholds his affection. As the fanfiction progresses, Trunks could start to rebel against Vegeta's ways, and they would have to navigate through their differences and find a middle ground.
One reason could be that Jacob realizes they have different life goals. Bella is often so focused on Edward and the vampire world, and Jacob may feel that his werewolf life and values are too different from hers to have a romantic connection. For example, he might want a more traditional werewolf life with his pack, while Bella is constantly drawn to the dangerous and alluring vampire lifestyle.
One possible reason could be a change in character development that diverges from the original show's trajectory. Maybe in the fanfiction, the writer wants to explore new relationship dynamics where Castle has different feelings. Another reason might be to create conflict for the plot. By having Castle not love Beckett, it can lead to a series of events that test their partnership in a different way.
One possible reason could be a change in character growth. Maybe one of the characters has evolved in a way that makes them incompatible with their former lover. For example, they might have developed new interests or values that conflict with those of their partner.