Maybe he realizes that the shinobi world is too corrupt. There could be political intrigue and power struggles that go against his ideals. He has always fought for peace and unity, but if he sees that the system is beyond repair, he might choose to quit. Also, he could have a family situation that requires him to step away from the shinobi life. For instance, if he has a child with special needs that demands his full attention.
One reason could be the excessive toll on his personal life. He might have realized that being a shinobi means constantly sacrificing his time with family and friends. For example, he missed out on important moments in his son's life. Another reason could be disillusionment with the shinobi system. He may have seen the corruption or the never - ending cycle of violence within it and decided he no longer wanted to be part of it.
One possible reason could be the overwhelming stress. Being Hokage means handling a lot of political, social and security issues in the village. Naruto might just feel that he can't take it anymore and decides to quit in the fanfiction. For example, dealing with the constant threats from other villages and internal disputes within Konoha can be mentally and physically exhausting.
One reason could be to explore a different side of Naruto's character. In the original series, he's so warm - hearted, but by making him cold and distant, the writer can show how different events could have shaped him in an alternate way. Maybe he faced more hardships or betrayals that led to this change.
One possible reason could be that he gets tired of the constant fighting and bloodshed. He might have seen too many friends get hurt or die, and it just becomes too much for him to bear. Another reason could be that he discovers a new purpose in life outside of being a ninja, like focusing on family or helping the village in a non - combat way.
One reason could be that she might have faced too much trauma from battles. Constant fighting, seeing friends get hurt or die, could have taken a toll on her mental health, leading her to quit.
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 blamed for some trouble he didn't actually cause. For example, if something went missing in the orphanage and the caretakers wrongly thought Naruto took it.
One possible reason could be some kind of dark magic or jutsu that has affected his family's memories. Maybe an enemy cast a spell specifically to make his family forget about him to weaken him.
One possible reason could be that he's framed for a crime he didn't commit. Maybe someone powerful wants to get rid of him and plants false evidence. For example, they could make it seem like he attacked a village ally when in fact he was defending it from a hidden enemy.
One reason could be that Naruto has been through a lot and is emotionally drained. He might see Tsunade as a source of comfort and stability, so he calls her mom. His coldness could be a defense mechanism to hide his vulnerability. Another reason could be that he has seen how Tsunade cares for others in the village, and he subconsciously wants that kind of motherly love for himself.