Perhaps there are power - hungry members in the guild who see Lucy as a threat. They might spread false rumors about her being too powerful and uncontrollable. So, they manage to convince the guild master to kick her out. In fanfiction, it's often about creating drama and conflict, and this would be a great way to do that.
One possible reason could be a misunderstanding. Maybe someone framed Lucy for something she didn't do, like stealing a powerful magic item from the guild. Another reason might be that she accidentally broke a very important and unbreakable guild rule without realizing it. For example, if there was a rule about not using a certain type of dark magic, and she unknowingly used it to save a friend.
It could also be related to her family background. If it's revealed that her family has some sort of dark past or connection to an enemy of Fairy Tail, the guild might feel that having her around is too risky, even though they know Lucy as a kind - hearted person. This would create an interesting plot where Lucy has to prove herself and clear her family's name.
Maybe she just lost interest in the Fairy Tail world. Fanfiction can be a passing fancy, and her enthusiasm might have waned over time.
One possible reason could be that she feels she has outgrown the guild. Maybe she wants to explore new magic on her own and believes the Fairy Tail environment has become too comfortable and is hindering her growth.
Perhaps Natsu is under the influence of a powerful and dark magic that makes him lose control of himself and accidentally hurt Lucy. It could also be that Natsu is trying to protect Lucy from something but his actions end up causing her harm instead. Like if he tries to keep her away from a dangerous situation by pushing her too hard and she gets injured.
In a fanfiction, there could be a situation where Lucy is forced to side with a group that Natsu is against. This would make Natsu see her as an enemy. Or perhaps there is a prophecy that Lucy is involved in that Natsu misinterprets as something bad. He might think she is a threat to the guild or his friends. Also, if there is a love triangle situation where Lucy chooses someone else over Natsu in a way that really hurts his feelings, that could be a reason for him to start hating her.
One possible reason could be a misunderstanding. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical item from the guild. And the guild, without proper investigation, decided to kick him out. Another reason might be his recklessness. If his actions put the guild in great danger too many times, the guild masters could decide that he's a liability and kick him out.
Happy would be sad and confused. He doesn't like to see his friends in trouble. He would fly around trying to gather information from different guild members. He would also stay by Lucy's side, trying to cheer her up. And all of her other friends in Fairy Tail would at least have some doubts about the decision. They know Lucy's character, and her sudden expulsion would make them question whether the guild made the right choice.
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.
Maybe he got into too many fights. Natsu has a short temper and if there were other students in the academy who provoked him or if he was standing up for someone, he might have engaged in brawls that the academy doesn't tolerate. In a place like Fairy Tail Academy, they might have strict rules about student behavior regarding fighting.
One possible reason could be that he's framed for a crime he didn't commit. Maybe someone planted false evidence to make it seem like he was endangering the guild. Another reason might be that his power got out of control and the guild, fearing for the safety of its members, made the tough decision to kick him out.
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.