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 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.
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.
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.
In fanfiction, the Naruto and Sasuke bond is often explored in many ways. Sometimes it's about their unwavering loyalty to each other despite their differences. Naruto is always trying to bring Sasuke back to the village, showing his deep friendship and belief in Sasuke's goodness.
It could also be due to Sasuke's secrecy. If Sasuke is constantly hiding things from Naruto, like his real intentions or new powers he's acquired, Naruto would naturally start to feel that he can't trust him. Trust is built on honesty and openness, and if Sasuke is being cagey, Naruto's trust in him will fade.
One possible reason could be his deep concern for Naruto. Despite their complex relationship, Sasuke might be worried about Naruto's well - being after all they've been through. He could be silently guarding him while he sleeps, a sign of his unspoken affection and protectiveness.
One possible reason could be that he overestimated his abilities during a battle in the Wave arc. Maybe he took on an enemy that was much stronger than he thought and got fatally injured. Another reason could be that he sacrificed himself to save Naruto or someone else. It's also possible that there was some sort of betrayal within the group they were with, and Sasuke was caught off - guard and killed.