One reason could be the trauma he experienced during the Wave mission. He might have seen the darker side of the ninja world, like the cruelty of Zabuza and Haku's situation. This could have made him more guarded and seemingly cold as he realizes the harsh reality.
In the fanfiction, perhaps Naruto was betrayed by someone in Wave. Maybe a villager he thought he could trust turned on him. This act of betrayal could have led him to close off his emotions and become cold to protect himself from further hurt.
Another possibility is that Naruto witnessed the powerlessness he felt during the Wave events. He couldn't save everyone, and this feeling of inadequacy might have changed his once - warm personality. He might think that being cold and more calculating would make him a stronger ninja in the future.
In some fanfictions, it could be due to a powerful emotional shock. Maybe seeing a friend in great danger at Wave, like Sasuke almost dying, could trigger an unexpected connection to the Sharingan. Since Naruto has a strong bond with Sasuke, this extreme situation might force his latent Uchiha - like powers (in the context of the fanfic) to surface, awakening the Sharingan.
It could be due to a new jutsu he's trying to master that has some side effects on his emotions. Maybe it's a powerful ice - based jutsu that's making him cold in nature as well.
Another reason could be the influence of a powerful and dark entity. If Naruto is somehow possessed or influenced by a dark chakra or a malevolent spirit, it might change his personality completely. He could start to push away people who care about him because the entity wants him to isolate himself and be more vulnerable to its control.
One possibility is that there's some ancient chakra or energy in Wave that interacts with Naruto's own chakra. The unique environment of Wave, with its water and nature elements, might have caused his Mokuton to awaken. Another reason could be that Naruto has some kind of connection to the First Hokage that gets revealed during the Wave mission, and that connection enables the Mokuton awakening.
It could be due to a great tragedy. For example, if all his friends were killed and he was filled with rage and revenge. In that state of mind, he might become a killing machine, just focused on destroying those who wronged him.
It could be related to some sort of external influence, like a curse or a dark chakra that has infected him. This could be slowly changing his personality from the warm - hearted Naruto we know to a cold - hearted version. For example, if there was a powerful enemy who placed a curse on him that targeted his emotions.
One possible reason could be a sense of being undervalued. Maybe he felt that his teammates didn't fully appreciate his efforts during the Wave mission. For example, if Sasuke got most of the credit, Naruto might feel pushed aside.
Maybe he was protecting someone in Wave. He's always the self - sacrificing type, so he could have put himself in harm's way to save a villager or a friend.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.
It could be due to a personal loss that has hit him really hard. Maybe he lost someone very close to him, like Jiraiya all over again, but this time it has broken him in a way that he shuts everyone out. He might also be under the influence of a new power that he doesn't fully understand yet, and this power is affecting his emotions, making him cold as a side - effect until he learns to control it.