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.
It might be due to a curse or a blessing from an ancient being. There could be a hidden entity in the Naruto universe that decided to give Naruto this unique trait. Or perhaps it's a side - effect of a powerful experiment or a forbidden technique that he was exposed to. This would add a whole new layer of mystery to the fanfiction.
Perhaps it has to do with his strong willpower. Naruto has always been known for his determination. In this fanfiction, his mental strength could have manifested in a physical way, blocking out pain. Another possibility is that there was some interference with his chakra system that disrupted the normal pain receptors. This could be caused by a strange chakra - based artifact or a new type of training he underwent.
His fighting style could become more aggressive. Without the fear of pain, he might be willing to use more extreme techniques that would otherwise be too dangerous because of the pain they cause to the user. He could push his body to the limit more often.
One possible cause could be his sense of failure. Naruto always wants to protect his friends and village, so if he fails in a mission or is unable to save someone, it can cause him great pain.
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.
He could feel it through a shared chakra link. Since they are related and Minato sealed the Nine - Tails in Naruto, there might be a latent connection that gets activated in the fanfiction, allowing him to sense Naruto's pain on a physical and emotional level.
Well, in this 'Naruto can't feel pain' fanfiction, a possible plot development is that Naruto's overconfidence due to not feeling pain could lead to him making mistakes. He might underestimate his opponents and end up in life - threatening situations. Also, the medical ninjas in the village might be very curious about his condition and try to find a way to either reverse it or use it for medical research. And his relationships with his comrades could be strained as they struggle to understand his new - found recklessness.
Another reason could be related to his relationships. Maybe in a fanfic, Naruto has a falling out with his friends like Sasuke or Sakura. This emotional hurt can be just as painful as physical harm. The author might use this to show how Naruto grows and overcomes such internal struggles.
The outcome in this fanfiction could be a draw. Naruto and Hinata would put up a great fight against Pain. Naruto would be all fired up, using his Rasengan and other ninjutsu. Hinata would be there, protecting Naruto's blind spots with her gentle fist techniques. But Pain is no pushover. His ability to control gravity and rain down destruction could keep Naruto and Hinata at bay, leading to a situation where neither side can fully defeat the other.
Naruto could be afraid of being seen as weak. In the ninja world, weakness is often frowned upon. He has a reputation to uphold, especially in front of his family. He doesn't want them to think that he can't handle his problems, so he hides his pain. He also might be trying to be strong for his family. If they see him in pain, it could affect their morale. So, he pretends that everything is okay to keep their spirits high and to give them the impression that they can rely on him no matter what.
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.
Well, if Naruto doesn't feel pain in the fanfiction, it could lead to him taking more risks in battles. He might be more fearless and charge into dangerous situations without hesitation. This could change the way battles are fought and won. For example, he could endure attacks that would normally incapacitate him and turn the tables on his enemies.