Maybe he has been through some extremely traumatic event. For instance, if he lost all his friends in a single battle, it could be so overwhelming that he shuts down his emotions to cope.
It could be the result of some dark jutsu being used on him. A powerful enemy might have cast a spell that drains his emotions, leaving him a shell of his former self. Another possibility is that he has made a pact or deal with some dark entity in order to gain power, and as a side effect, he loses his emotions.
Perhaps he has seen too much of the cruel side of the ninja world all at once. Like if he witnessed massacres or the true nature of some powerful and corrupt ninja clans. This could make him so disillusioned that he decides to cut off his emotions as a form of self - protection. He might think that by not feeling, he can't be hurt or disappointed anymore.
One reason could be false accusations. If Naruto is wrongly accused of a major crime in the village and no one believes him, he might start to hate Konoha.
Perhaps an ancient Uchiha clan member whose soul was wronged in the past. Since the Uchiha have a lot of dark secrets, their grudge could target Naruto for some reason. It could be related to the Sharingan or some long - forgotten Uchiha feud.
It could be an old enemy who has come back with new and more terrifying powers. For instance, Orochimaru might return with some kind of jutsu that can directly target Naruto's mind and create unfathomable fears in him.
It's interesting because it allows for new relationship dynamics. His friends would have to deal with this new Naruto in different ways. For example, Sasuke might be more intrigued or concerned. Also, the plot can take unexpected turns. Instead of Naruto being the driving force of hope and motivation, he could be a more reserved and calculating character, which can lead to some really complex and engaging storylines.
One key element could be a character who is like an emotionless weapon. In Naruto fanfic, this might be a new character or an existing one with a different twist. For example, they could be trained from a young age to be a killing machine without emotions. Another element could be the conflict this character faces in a world full of emotions like the Naruto universe. They might struggle to understand the bonds and values that other characters hold dear. Also, the plot could revolve around how this emotionless weapon - like character either changes or influences the Naruto world around them.
One common trait is extreme intelligence. They can analyze complex situations and jutsus very quickly. For example, they can predict an opponent's moves several steps ahead. Another trait is their detachment from emotions like fear or excitement during battles. They stay calm and collected, which gives them an edge. They also often have unique abilities that set them apart from regular characters in the Naruto world.
Naruto could use his explosive - natured jutsus like the Rasengan. If he was sparring with Kiba, a misdirected Rasengan could hit something explosive nearby and cause an explosion involving Kiba. For Issei, if they were in a battle together, Naruto's chakra - infused attacks might react with Issei's energy - based powers in an unexpected way and create an explosion.
It could be Orochimaru. He's always been interested in Naruto's power. He might use some new jutsu or experiment on Naruto to make him go berserk so he can study his power in that state.
In some cases, it could be a new and powerful enemy. This enemy might have the ability to manipulate chakra in a way that affects Naruto's mind. For example, they could send out waves of negative chakra that slowly erode Naruto's mental state. Over time, these attacks make Naruto go insane as he struggles to fight against this unseen and insidious force.
Madara Uchiha could be the cause. Even if he's dead, his influence could still be around. Maybe he left behind some sort of genjutsu or curse that affects Naruto's mind and drives him crazy. After all, Madara was always planning something big and Naruto would be a great pawn if he's out of control.