In the fanfiction, Sakura could be jealous because she has an ego. She might have thought that Naruto would always be there waiting for her. But when he falls for Hinata, it bruises her ego. Additionally, she may have seen Naruto as her safety net, someone who would always support her no matter what. Losing that special status to Hinata makes her envious and jealous.
Perhaps Sakura is jealous because of her own insecurities. She might be comparing herself to both Naruto and Ino. Naruto has his own charm and strength, and Ino has her grace and popularity. Sakura, who might be struggling with her own self - worth, could see their interactions and feel envious, thinking that they are better off without her or that they have something she doesn't.
In fanfiction, Naruto might be jealous because he thinks that Kiba has something he doesn't when it comes to Hinata. For example, Kiba could be more confident around Hinata, while Naruto is still a bit shy about his feelings for her. This contrast could fuel Naruto's jealousy.
In such a fanfiction, Hinata might be jealous of Sakura because Sakura could be more popular or confident. Maybe Sakura has skills that Hinata wishes she had, like better combat skills or more outspokenness. Also, if there's a love interest involved, and the guy seems to pay more attention to Sakura, it would make Hinata jealous.
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.
One possible scenario in this fanfiction could be that Sakura realizes her long - hidden feelings for Naruto when she sees Hinata getting closer to him. She might start to act out in small ways, like being a bit more snippy with Hinata during team meetings or training sessions. However, she also has to deal with her own sense of pride and the fact that she once rejected Naruto.
It could end with Sakura fully accepting Hinata and Naruto's relationship. She realizes that her jealousy was unfounded and that she is happy for them. She might even become one of their biggest supporters.
In such a fanfic, Sakura might start acting out of character. She could become more distant from Naruto and Hinata. Maybe she'd try to prove herself better than Hinata in front of Naruto, like showing off her ninja skills more often.
One possible plotline could be that Sakura realizes Hinata has a closer relationship with Naruto than she does. So, she starts to feel jealous. For example, she might notice how Naruto always seems to have a soft spot for Hinata's gentle nature. This could lead to Sakura trying to prove herself better in Naruto's eyes, like training extra hard or taking on more difficult missions.
Some people might write it because they prefer other characters over Naruto, Hinata, and Sakura. They could be trying to promote their favorite characters by putting these three down in their fanfiction.
Perhaps it's because in the main Naruto storyline, Hinata has so many unspoken feelings for Naruto. Fanfiction gives her a platform to express those feelings more openly and in various scenarios that she may not get in the official story.