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.
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.
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.
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Chapter 601, "Going Home", is the clearest early answer because it turns Gu Qiang's situation into a concrete plot event. The chapter gives readers a real answer by opening on Gu Qiang was in a very pleasant mood. and ending on They finally had a day off, better let him be..
Chapter 582, "Fleeing Hastily", is the clearest early answer because it turns Team's situation into a concrete plot event. The chapter gives readers a real answer by opening on Luo Xuan pushed her away, disappointment filling his eyes.
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