One possible reason could be that Naruto has seen the dark side of the shinobi world. He might have witnessed too much death and destruction in battles, and this has made him disillusioned. For example, if he lost many friends in a major war, he could start to question the whole concept of being a shinobi.
Another reason could be that he has had a major ideological shift. He might have met someone who introduced him to a new way of thinking that opposes the shinobi system. This person could have shown him that the shinobi way is often based on power struggles and political games, which Naruto no longer wants to be a part of.
Maybe he discovers other dreams or goals. He could find that there are other ways to protect the village and his friends that don't require him to be Hokage. He might be more interested in exploring the ninja world, learning new jutsus, or even teaching the younger generation about ninjutsu rather than taking on the political and leadership role of Hokage.
One possible reason could be the fear of losing his loved ones. In the original Naruto story, many ninjas sacrifice a lot, including their lives. Naruto might have seen this and decided he doesn't want to put himself in such a position.
It could be that Sakura has done something that has disappointed Naruto deeply. This could range from her making a decision in a mission that Naruto didn't agree with, to her not being there for him during a difficult time. These actions might have gradually eroded his love for her.
One possible reason could be that he has seen the darker side of the ninja world. Maybe he witnessed too much death and destruction during his missions and decided he no longer wanted to be part of it. Another reason might be that he has other dreams that he realizes are more important to him than being a ninja. For example, he could have a passion for art or music that he wants to pursue instead.
One possible reason could be that he feels betrayed by Konoha. Maybe he has uncovered some dark secrets about the village that make him lose faith in it.
His relationship with Sasuke would be greatly affected. Sasuke, who has always been somewhat defined by his rivalry with Naruto as a shinobi, might not know how to interact with Naruto anymore. They could grow apart as Sasuke still pursues the shinobi path while Naruto abandons it.
One possible reason could be past decisions made by Sarutobi that Naruto didn't agree with. For example, Sarutobi's handling of the Uchiha situation. Naruto, who values friendship and fairness deeply, might have seen this as a sign of Sarutobi's inability to do what's right, thus leading to his distrust.
One reason could be the excessive toll on his personal life. He might have realized that being a shinobi means constantly sacrificing his time with family and friends. For example, he missed out on important moments in his son's life. Another reason could be disillusionment with the shinobi system. He may have seen the corruption or the never - ending cycle of violence within it and decided he no longer wanted to be part of it.
Maybe they don't like fanfiction in general. It could be that they prefer the official, canon Naruto story. Fanfiction can vary greatly in quality, and they might have had bad experiences with some poorly written Naruto fanfictions in the past, which made them dislike the whole concept of Naruto fanfiction.
One possible reason could be past betrayals. Maybe he has been let down by people he thought were friends in the fanfiction world. For example, if someone he trusted shared his secrets which led to him getting into trouble.