Perhaps there were new characters or magic elements introduced that Natsu wasn't familiar with. Fanfictions often bring in unique elements that can catch characters off guard.
Maybe it was a complex plot twist that left Natsu confused. Sometimes the storylines in fanfictions can get really convoluted.
It could be that the events were introduced too abruptly or without enough background for Natsu to understand. Maybe the author wanted to build suspense this way.
Maybe Natsu hates Fairy Tail fanfiction because he doesn't like how some fanfictions distort the original story or the characters. In the original Fairy Tail, he has a clear personality and relationships, and fanfictions might change that in ways he wouldn't like.
There might be a lot of bad writing in the fanfiction that involves Natsu. Poorly written dialogue, actions that don't make sense for his character, or just a general misrepresentation. If the fanfiction is full of these elements, it could give the impression that Natsu hates in that fictional world created by the writer, even though it's not true to his real character.
One possible reason could be a past trauma. Maybe he witnessed something really horrible during a battle or an incident at school that shocked him so much he lost his ability to speak. For example, if he saw a friend in great danger and was powerless to help immediately, it could have had such a psychological impact on him.
Well, in some fanfictions, the author might want to explore a different side of Natsu. Maybe they think it adds drama. Natsu could be written as a jerk because of some past trauma that's not shown in the original series, which makes him lash out at everyone in the fan - created story.
One reason could be that the author wants to create an alternate universe where Natsu has a different path. Maybe in this fanfiction, he has a prior commitment to another guild or group that prevents him from joining Fairy Tail.
Often, it could be because of a false accusation. Maybe someone framed Natsu for a crime he didn't commit, like stealing a powerful magical artifact from the guild. So, to maintain peace and order, the guild leaders had no choice but to banish him.
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.
Perhaps in the fanfiction's plot, Natsu makes a big mistake that endangers Lucy or her friends. Let's say he recklessly takes on a powerful enemy without a proper plan, and Lucy's friends get hurt as a result. This might cause her to hate Natsu for his irresponsible behavior.
He could kidnap her because he believes she's in immediate danger. Natsu is very impulsive and might not have time to explain things properly, so he just takes her away to keep her safe.
In fanfiction, Lucy might want Natsu because of their strong bond. They've been through many adventures together. Natsu is always there to protect her, and his brave and fun - loving nature attracts Lucy.