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.
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.
Chapter 249, "Maxing Out Wilderness Survival Skills!", is the clearest early answer because the violence around Survival Skills becomes final instead of hypothetical. The chapter turns that into something concrete by moving from Jiang Yan's body subconsciously shivered twice.
Chapter 466, "Are There Really Wild Boars?!", is the clearest early answer because it turns Bai Lu's situation into a concrete plot event. Early on, The lady sure acts like a lady, even in 'Wilderness Survival' she....
Chapter 446, "Actually Sending Jiang Yan to Pick Up Trash?!", is the clearest early answer because it turns Dad's situation into a concrete plot event.
Chapter 388, "All the Rich People in Jing City Are in Jiang Li's Live Room", is the clearest early answer because it pushes Rich People into a new phase or setting. Early on, what, what do you plan to do?" Gu Yu's false identity was never meant....
Chapter 292, "Coming and Going, Gathering and Parting, Human Nature", is the clearest early answer because it turns Eternal Life Talisman's situation into a concrete plot event. A useful checkpoint is the way the scene starts with Jiang Li slowly lowered his hand.
Chapter 411, "You Really Hold a Strong Grudge", is the clearest early answer because it turns Hua Country's situation into a concrete plot event. It opens with The lead director, who understood the entire process, had an...
Chapter 233, "Individual, Ten, Hundred, Thousand... Million, Ten Million, Damn!!!", is the clearest early answer because it turns Tianyi's situation into a concrete plot event. The chapter turns that into something concrete by moving from [What the hell, what's this about life span ending??
Chapter 338, "The Adulterer is Far on the Horizon, Yet Close Before the Eyes", is the clearest early answer because it turns Li's situation into a concrete plot event. Early on, Tang Jinyu, dressed in a light apricot Cheongsam, sat indifferently....
Chapter 210, "Are All of Jiang Li's Fans Hidden Big Shots?", is the clearest early answer because it turns Miss Jiang's situation into a concrete plot event.
Chapter 463, "Even if it's a Wild Boar, I'm Willing!", is the clearest early answer because it turns Jiang Li's situation into a concrete plot event. The chapter turns that into something concrete by moving from The evening glow gradually bathed Dragon Ridge Island in a dazzling...