It could be about the deep bonds and loyalty within the 'Fairy Tail' universe in the context of fanfiction. Maybe it focuses on characters making a solemn vow not to leave each other, which is a common and powerful theme in 'Fairy Tail' as the guild members are like a family.
The loyalty aspect. In 'Fairy Tail', loyalty among the guild members is a huge part of the story. So a fanfiction centered around a promise to never leave plays on that strong theme and makes it interesting.
If Lucy never joined Fairy Tail, the friendships and relationships among the characters would form in a very different way. Also, many of the story arcs might not unfold as they do in the original plot.
It could lead to a change in the guild's dynamics. Naruto has his own unique combat style and abilities. His departure might make the guild realize the need to diversify their fighting strategies. For example, they might start to look for members with skills similar to Naruto's ninja techniques.
He could leave because of a misunderstanding. Maybe he overhears something wrong and thinks the guild doesn't trust him or value him anymore. So in a fit of anger and pride, he decides to leave.
A possible reason could be personal conflicts within the guild. Lucy might have an issue with some of the guild's rules or how certain things are run. Juvia could be following Lucy for some reason, like loyalty or because she also has her own issues with the guild. For instance, if there are tensions between Juvia and other members over Gray.
Maybe they leave because they want to search for answers about their pasts. Happy might want to know more about his origins, and Lucy could be seeking something related to her family's history.
Perhaps some dark mages who have a vendetta against Fairy Tail. They target Lucy and Gray because they think that these two are easier to take down outside of the protection of the guild. They might want to use Lucy and Gray as bait to draw out other members of Fairy Tail.
One possible reason could be a conflict of power. Naruto has his own set of powers from the Naruto universe, like chakra and jutsus. In Fairy Tail, magic is the main power source. Maybe his powers were seen as a threat or too different, causing the guild to force him out. For example, if his chakra-based attacks were disrupting the magical balance in Fairy Tail's world.
Maybe there are some dark mages who are after the magic that Lucy and Levy possess. As they leave Fairy Tail, they lose the protection of the guild, and these dark mages could target them. They would be antagonists as they try to steal the girls' magic or harm them in some way.