Perhaps they leave because of a disagreement within the guild. Maybe there's a new policy or a decision made by the guild master that they don't agree with. For example, if the guild decides to get involved in a war that they think is unjust, they might choose to leave rather than participate against their beliefs.
One possibility is that they leave to explore a new land filled with unknown magic. They might be on a quest to find a powerful magic item that could help Fairy Tail in the future.
The OC could have accidentally said or done something that made Natsu feel unwanted or unappreciated in Fairy Tail. For example, if the OC is a new member and got more attention than Natsu in a certain situation, it might have bruised his ego and led to his decision to leave.
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.
One possibility could be that they leave to seek greater magical power. Maybe they've heard of a mysterious land filled with powerful magic and they decide to embark on a journey together. They leave Fairy Tail behind in order to become stronger and return to protect their guild in a more powerful way.
One possible reason could be to seek new adventures. They might feel they've reached a point in Fairy Tail where they've done all they can and want to explore new magic and challenges elsewhere. For example, they could have heard of a far - off land with a different type of magic that they are eager to learn about.
Well, 'leave the pieces novel' might refer to a concept in writing or reading novels. Maybe it implies that the author intentionally leaves some parts of the story, like pieces of a puzzle, for the reader to figure out on their own. It could add an element of mystery or make the reader more engaged in the interpretation of the text.