The different power scales. Naruto has his own progression of power through training and ninja ranks. Superman's power is on a completely different level like being able to move planets. In fanfiction, it might be hard to balance their powers in a way that makes sense to Naruto fans, so Naruto might seem to 'hate' it.
One reason could be that some fans are very purist about the original Naruto story. They might feel that fanfiction often distorts the characters or the plot in ways that go against the essence of what Naruto is all about. For example, changing a character's personality completely to fit a romantic or dark and edgy narrative that doesn't align with the source material.
Perhaps in fanfiction, Boruto is associated with a group or ideology that Naruto opposes. This could cause Naruto to view Boruto unfavorably. Also, if there are false rumors or misinformation spread in the fanfiction world about Boruto, Naruto might believe them and start to hate Boruto until the truth is revealed. And sometimes, it's just a way for the writer to make an interesting and unexpected plot twist.
A possible reason could be jealousy. If Naruto had a sibling who was always favored by their parents, he might grow to hate his family. He would feel like he was always in the shadow of his sibling and that his family didn't care about him as much.
One reason might be that he doesn't like how fanfiction can change the relationships between characters. For example, in the original, his relationship with Naruto is based on training and mentorship, but fanfiction could make it something completely different.
One possible reason could be a misunderstanding. Maybe some false information was spread about his mother, making Naruto believe she had abandoned him. Another reason could be related to the secrecy around his mother. If he grew up feeling alone and not knowing about her, he might develop resentment.
Perhaps he thinks fanfic undermines the integrity of the original Naruto story. The official story has a certain narrative and character development, and fanfic can deviate from that significantly. When it comes to his connection with Naruto, any deviation in fanfic might be seen as a negative. For example, if a fanfic changes the way he mentors Naruto, it could be something he wouldn't like.
One reason could be that it's simply not his genre. Naruto is used to his own action - packed, ninja - themed story. Fallen angel fanfiction is likely to be more on the supernatural and spiritual side, which he may not be interested in.
Perhaps the art styles play a role. Naruto has a distinct art style that some people are really drawn to, and when they look at Fairy Tail, they just don't like the visual aspect as much. Also, the story themes in Naruto are often darker and more about political intrigue and personal sacrifice, while Fairy Tail is more about friendship and adventure. Fans of Naruto's darker themes might not like Fairy Tail's more upbeat nature.
One possible reason could be that he's had a bad experience with the taste. Maybe the ramen was made wrong one day and it left a really bad impression on him.
One possible reason could be that Naruto might feel Kakashi is too strict with his training. Maybe Kakashi's methods are too harsh for Naruto, making him frustrated and gradually developing dislike.