There could be a situation where Sasuke has made a choice that Naruto simply cannot forgive. For instance, if Sasuke were to harm innocent people in the pursuit of his goals. This would shatter the bond they once had as Naruto values the lives of all. And with Sasuke showing no remorse for such actions, Naruto would be forced to sever their bond.
It could be due to external influence. Perhaps an enemy has manipulated Sasuke into believing Naruto is a threat to his goals. Also, Sasuke might have felt betrayed by the village in some way and Naruto, being associated with the village, has become a casualty of that. Naruto, on his part, may not be able to tolerate Sasuke's new, potentially dangerous path.
One possible reason could be a major ideological difference that has grown over time. Maybe Sasuke has adopted a more extreme view on protecting the village or avenging his clan that Naruto can't support. Another reason might be a misunderstanding that has festered. For example, if Sasuke was misinformed about Naruto's actions and intentions and has cut off ties because of it.
Well, it completely flips their relationship. Instead of being comrades or rivals with an underlying bond, they become estranged. Naruto may start to see Sasuke as a lost cause or an enemy. Sasuke, in turn, would be on his own path without Naruto's influence.
One reason could be a fundamental difference in their goals. Maybe Naruto decides that Sasuke's pursuit of power in a certain way goes against his own beliefs of protecting the village and its people. For example, if Sasuke were to side with an enemy that Naruto sees as a threat to Konoha, Naruto might break the bond to protect his home.
Another reason might be related to their different ideologies within the ANBU. Naruto may have believed in a more collaborative and friendly approach within the organization, while Sasuke might have been more focused on individual achievement and power. This fundamental difference could lead to Naruto hating Sasuke in the fanfiction.
One possible reason could be to protect Hinata. Maybe Sasuke has unknowingly made a move or decision that Naruto perceives as a threat to Hinata, so he scares Sasuke to make him back off.
In the fanfiction, Naruto could stop chasing Sasuke because he has a moment of self - realization. He might understand that his own identity and purpose in life are not solely defined by bringing Sasuke back. He could find new goals and dreams that don't involve Sasuke. Also, there could be a major event in the ninja world that makes Naruto shift his focus away from Sasuke, like a new and powerful enemy that threatens the entire village or region.
One possible reason could be that Sasuke might have gone too far in his pursuit of power even after the war. He could have been a threat to the newfound peace in the ninja world, and Naruto, as the protector of peace, might have felt he had no other choice.
One possible reason could be that Naruto realizes Sasuke is too far gone in his quest for revenge. Naruto might see that no matter how much he tries, Sasuke won't change his ways, so he gives up. For example, if Sasuke keeps attacking innocent people in his pursuit of power to defeat Itachi's enemies, Naruto might finally lose hope.
It could also be due to a misunderstanding. Perhaps there is some misinformation spread about Sasuke in the fanfiction, like false rumors that he has a hidden and malevolent plan. Naruto, being someone who values their friendship deeply, would be scared at the thought of Sasuke turning into an enemy because of these untrue stories.
One reason could be Sasuke's thirst for power. If he believes Naruto is the only obstacle in his path to ultimate power, he might be driven to kill him. In some fanfictions, Sasuke may be influenced by a dark entity that twists his mind into thinking Naruto is a threat that needs to be eliminated.
Maybe Naruto gets tired of always chasing after Sasuke. He has his own life and dreams too. He might think that he has sacrificed enough of his time and energy on Sasuke, and it's time to focus on himself and the village. He could start to believe that Sasuke has made his own choices and he can't keep forcing Sasuke to come back.