Well, it might be that Naruto is in a state of extreme chakra depletion. Chakra is closely related to a ninja's physical and mental state. If his chakra is drained to a very low level, it could lead to a loss of feeling. It's like when a person is extremely exhausted, their senses can become dull. And in Naruto's case, it could be magnified because of his connection to the Nine - Tails' chakra.
One possible reason could be a powerful jutsu gone wrong. Maybe he was hit by an enemy's attack that was specifically designed to numb his senses. Another reason might be some kind of psychological trauma. For example, if he witnessed something extremely horrible, his mind might have shut down his ability to feel as a form of self - protection.
It could be due to a curse. There are many dark curses in the Naruto universe. If Naruto was cursed, it could affect his ability to feel. Perhaps an evil shinobi put a curse on him to weaken him. Also, it might be related to some experiment. If he was experimented on by Orochimaru or other rogue scientists, they could have messed with his nervous system and caused this.
Maybe he's been cursed by a powerful enemy. In many fanfics, an enemy might use a dark jutsu to take away Naruto's ability to feel. It could be a way to weaken him both physically and mentally.
One possible reason could be some kind of dark magic or curse that has been placed on him. Maybe an enemy wants to weaken him by taking away his emotions so he can't fight with his usual passion.
It could be Orochimaru. He's always been interested in experimenting on strong ninja. Naruto, being the main character, would be an interesting target for Orochimaru to mess with and cause this condition.
One possible reason could be that he was severely injured in a battle. Maybe he got his legs broken or had some internal damage that affected his ability to move them. For example, if he fought against a really powerful enemy who used a jutsu specifically targeting his legs, it could leave him unable to walk.
With the new characters introduced in the fanfic, if there are any, Naruto's lack of feeling could create an interesting dynamic. They might either be intrigued and want to help, or they might take advantage of his situation. And for Naruto, he would have to rely on others in a different way than before, which could either strengthen or strain his relationships depending on how others react.
One possible reason could be the stress of his ninja duties. He has so many missions and responsibilities that his mind is constantly racing, making it hard to relax and fall asleep.
One reason could be some sort of special jutsu or power - up. Maybe he got a new form or ability that suppresses pain. For example, it could be related to his Sage Mode or something new entirely.
Maybe he had a bad experience as a kid. Like he almost drowned once, and that made him too scared to learn how to swim properly.
His lack of feelings could also make him vulnerable to manipulation. Villains might try to use him as a weapon since he doesn't have the emotional restraint that he used to. This could create a complex web of events where Naruto's friends have to rescue him from the clutches of the villains while also dealing with the threat he poses.
One possible reason could be a different upbringing in the fanfiction's alternate universe. Maybe he was raised in isolation or by people who influenced him to suppress family feelings. For example, if he was raised by a group that emphasized self - reliance above all else, he might grow up thinking family bonds are a weakness.