One common theme is the exploration of unrequited love. Sakura's love for Sasuke is well - known, and when he leaves her for Karin, it delves into the pain of unrequited love from Sakura's perspective. Another theme is the idea of choosing based on power and usefulness. As Karin has certain powers that can be of help to Sasuke, it shows a theme of practicality in relationships.
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.
Sakura might be heartbroken. She's been in love with Sasuke for so long. She could initially go into denial, thinking it's just a temporary thing. Then she might get angry and vow to prove herself to Sasuke, perhaps by becoming an even stronger ninja.
One possible reason in this fanfiction scenario could be that Sasuke feels a stronger connection to Karin on a spiritual or power - related level. Karin has unique abilities that might attract Sasuke more than Sakura's medical ninjutsu. Maybe he sees in Karin a kindred spirit in terms of their past traumas and the way they've had to fight to survive. Also, Karin has always been very devoted to Sasuke, and he might be drawn to that unwavering loyalty.
One possible reason could be his long - standing feelings for Sakura. Despite his journey and time with Karin, his original connection with Sakura might have resurfaced. Sasuke has known Sakura since they were young, and those early memories and emotions could have influenced his decision.
Love triangles are a very common theme. Sasuke is often at the center of the attention of both Sakura and Karin, which creates a lot of tension and drama.
Another reaction could be that Sakura tries to follow Sasuke. She believes that she can change his mind or that she can be of help to him. She might leave her own responsibilities behind for a while to chase after him, thinking that their bond is strong enough to overcome whatever is making Sasuke leave.
There could be a fanfic where Naruto, Sakura, Sasuke, and Karin wake up with a hangover after a wild celebration in the Hidden Leaf Village. Maybe they had a party to celebrate a big victory against an enemy. Naruto would probably be the first to wake up, feeling groggy but still ready to face the day. Sakura, on the other hand, might be a bit grumpy and complain about the headache. Sasuke would be his usual stoic self, but even he couldn't hide the fact that he wasn't feeling his best. Karin would be using her medical skills to try and ease the pain for everyone. It could be a humorous fanfic with lots of banter between the characters as they deal with the after - effects of their drinking.
Well, it could be a story about their complex relationships. Maybe it shows how they interact in a post - war Naruto world. Sasuke might be in the middle of Sakura's love and Karin's lingering feelings for him.