It's mainly because fans love the original works and want to explore new storylines and possibilities with their favorite characters.
There are several reasons. Fans might be unsatisfied with certain plot points or want to give characters different endings. Also, it's a way for them to express their creativity and imagination based on the existing universes of these shows.
Well, in fanfiction, Lucy might be crying because she lost a fight against a powerful enemy. Maybe she felt she let her friends down. For example, if she was unable to protect someone important in the story, it could lead to her breaking down in tears.
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.
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.
One possible reason could be the difference in power systems. Naruto's world has its own unique ninja abilities like jutsus, while Fairy Tail has magic. In some fanfictions, the clash of these two different power concepts might make Fairy Tail fans dislike Naruto being involved. Maybe they think Naruto's abilities are too overpowered or not in line with the magic - based universe of Fairy Tail.
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 should not mess with Fairy Tail fanfiction because it is a creative space for fans. Fans pour their love for the series into these stories, and messing with them would be disrespectful to their efforts.
People like to watch Fairy Tail fanfiction because it allows them to explore different storylines and scenarios that the original series might not have covered. It gives fans a chance to see their favorite characters in new situations, like different romantic pairings or epic battles in alternate universes.
Naruto might be interested because he loves adventures. Fairy Tail is full of exciting adventures, just like his own life as a ninja. So, the fanfiction would offer him a chance to experience new kinds of adventures through the eyes of Fairy Tail characters.
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.