It could be that his new 'godlike' power has isolated him. People might be either in awe or afraid of him, and he may feel that he can't be close to others as before. So he acts coldly as a defense mechanism.
Well, at first, people around him would be very confused and try to reach out. But Naruto would just push them away. Then, his former friends might start to get frustrated with his coldness. Some might even turn against him thinking he has changed for the worse.
One possible reason could be a great betrayal. Maybe he discovers that someone he deeply trusted, like a close friend or a respected mentor, has been using him all along. This would shatter his trust and make him withdraw from people, thus becoming cold.
Perhaps he is trying to hide his true self. He may have a side of him that he doesn't want others to discover, like some special abilities or a different origin. So, being cold to everyone helps him keep his secrets and avoid getting too close to people who might figure him out.
One possible plot could be that Naruto has faced so many betrayals. Maybe his friends turn on him due to some false accusations. This shatters his warm - hearted nature, and he becomes cold as a defense mechanism. He starts to isolate himself and no longer trusts easily.
His friends would be hurt. They are used to the warm - hearted Naruto, and his coldness would make them feel rejected and confused. They might try to reach out to him but be constantly rebuffed.
One possibility could be that Naruto is drawn to boxing because of its physicality and the determination it requires. He might see it as a new way to test his strength and endurance outside of ninja battles. Maybe he starts training with a local boxing coach who sees his potential and helps him develop his skills.
It could be an evil wizard. There are many powerful and malevolent wizards in the Fairy Tail universe who might target Natsu for some reason and use dark magic to make him cold.
One possible reason could be a major betrayal. Maybe someone he trusted deeply, like a close friend or a mentor, turned against him. This would make him lose faith in people and become cold as a form of self - protection.