Well, one reason could be Sasuke's continued obsession with his revenge. He might be so focused on it that he ignores Sakura's feelings and concerns, which could lead to a fight. For example, if Sakura tries to persuade him to let go of his revenge and he refuses, it could spark an argument.
Maybe it's about Sasuke's secretive nature. He often keeps things to himself, and in a fanfic, Sakura could get tired of it. She might feel that he doesn't trust her enough to share his problems. So, she confronts him about it, and that turns into a fight.
Another possibility is jealousy. In a fanfic, there could be other characters who show an interest in either Sasuke or Sakura. If Sasuke gets jealous of someone getting too close to Sakura, or vice versa, it can easily lead to a fight between them.
In a fanfic, Sasuke's association with dangerous elements might make Sakura scared. For instance, if he teams up with a group of rogue ninjas. Also, his emotional state could play a role. If he is consumed by revenge and his anger is constantly boiling over, Sakura, who cares about him, might be scared of his self - destructive tendencies. Additionally, if Sasuke were to develop some sort of mind - control ability that he struggles to control, Sakura would surely be frightened.
One possible reason could be Sasuke's continuous absence due to his solo missions. He's always away, leaving Sakura feeling lonely and neglected in their relationship. This lack of emotional and physical presence can gradually erode the bond between them.
One possible reason could be that Sakura has finally grown tired of Sasuke's aloof and cold behavior towards her. She might have realized that she has been constantly chasing after him while getting little in return, so she decides to ignore him as a form of self - preservation.
One possible reason could be a difference in their views regarding the future of the village. Sasuke might have some extreme ideas that Sakura, as a more village - centered person, can't accept. For example, Sasuke may think about taking some radical actions to change the shinobi world order which Sakura believes would put the village at risk.
One reason could be that Sasuke really values Sakura's friendship and is afraid of losing it to someone else. Maybe she sees others getting closer to Sakura and feels left out.
One possible reason could be that he finally realizes Sakura's unwavering love for him. Throughout their time apart, he might have come to understand how much she truly cared, and now he regrets not being there for her. Also, he could have seen how she grew stronger on her own and feels he missed out on being part of that growth.
In the fanfic, it might be because of a misunderstanding. For example, Sasuke could be wrongly accused of something, and Sakura, believing the false rumors, is so disappointed in him that she refuses to talk. Or perhaps Sasuke accidentally said something really offensive during an argument, which made Sakura so angry that she just cut off communication.
There are fanfics like that. Sasuke might leave Sakura for Karin because of Karin's loyalty. Karin has shown unwavering loyalty to Sasuke throughout the series. She was willing to risk her life for him on multiple occasions. In contrast, Sasuke might think that Sakura's love is more of a childhood crush that she hasn't fully grown out of, and he may be drawn to Karin's more mature and selfless loyalty.
One reason could be that Sasuke is very popular and has a lot of fangirls. In the fanfic, Sakura's boyfriend might feel insecure because Sasuke has this charm that attracts everyone. Maybe Sasuke is always achieving great things and getting a lot of attention, which makes the boyfriend jealous as he might feel he can't compete with Sasuke in terms of popularity and achievements.
In the fanfic, it could be that Sasuke doesn't appreciate Sakura enough. She might pour out her love and support for him, but he doesn't return it in the way she hoped. Sakura may have expected a more reciprocal relationship where her feelings are acknowledged and valued. Since that's not the case, she starts to regret her choice of Sasuke.