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.
Well, it could be that in the fanfic, Sasuke has a special bond with Sakura. Even if it's just a platonic bond, Sakura's boyfriend might misinterpret it. He could see how Sakura looks at Sasuke or how she reacts when Sasuke is around. This could stir up jealousy as he fears losing Sakura to Sasuke, even if there's no real threat of that happening.
Sakura might directly confront her boyfriend. She could tell him that he has no reason to be jealous of Sasuke. For example, she could say that her feelings for her boyfriend are completely different from any admiration she has for Sasuke.
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.
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 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.
Well, Sasuke being jealous in this arranged marriage fanfic could lead to some intense moments. He could start training more vigorously to prove his worth to Sakura. Since he has a prideful nature, he wouldn't easily admit his jealousy. But his actions would speak louder. He might get into arguments with the elders who arranged the marriage if he feels they are not considering Sakura's feelings or his own connection with her.
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.
Perhaps Sasuke was testing Sakura's loyalty or strength. He may have thought that if she couldn't withstand his aggression, she wasn't worthy of being in his life. Also, his own inner demons, like the hatred he carried from his past, could have flared up and led him to almost kill her without fully realizing what he was doing.
Well, one possible reason could be that Sasuke's mistake might have put their team in great danger. For example, if his actions led to the enemy getting an upper hand during a battle, endangering not only his life but also those of his comrades like Sakura. Sakura, who values the safety of the team, would then resent Sasuke for his mistake.
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 because of Naruto. If Hinata has a closer relationship with Naruto than Sakura does in the fanfiction, Sakura could be jealous. After all, Sakura has her own feelings for Naruto.