Sakura's obsession with Sasuke. Naruto might be fed up with how Sakura always puts Sasuke first, ignoring the people around her who truly care about her, like Naruto himself. So he tells her off to make her realize her wrong priorities.
In this case, Sakura could be seen as the antagonist, at least in that particular moment when Naruto tells her off. Since Naruto is the one doing the telling - off, it's because of something Sakura has done or said that has pushed him to that point.
Well, it could be that Sakura makes a really bad decision during a mission, like endangering the whole team because she was being too overconfident. Naruto, who has grown tired of her reckless behavior, tells her off. He might say things like 'Sakura, you can't keep acting this way. You're putting all of us at risk!'
One possible plot could be that Naruto has finally had enough of Sakura's constant nagging or her one - sided focus on Sasuke. So he tells her off in a moment of exasperation, perhaps pointing out how she's been neglecting their friendship or the team spirit in pursuit of Sasuke.
Perhaps Sasuke has been ignoring Sakura's feelings for a long time in the fanfiction. Sakura decides she's had enough of his cold attitude. She tells him off for not being more considerate of her emotions. She might bring up all the times she's been there for him, and how he just brushes her off, and she demands some respect.
Obviously, Sakura and Sasuke are the main characters. Sakura, as we know from the Naruto series, has feelings for Sasuke. But in this fanfiction, those feelings might be overshadowed by her frustration with him. Sasuke, who has always been a bit of a lone wolf, might be surprised by Sakura's outburst. Their relationship, whether it's a friendship or something more complex, is at the center of this story.
He might target the elders first. They are often the ones who hold the traditional views and might be resistant to Naruto's new ideas. They could be the source of a lot of the problems Naruto is facing in the village.
He may try to talk to Sakura and understand her reasons. Naruto is a character who believes in the goodness of people. So, instead of getting angry immediately, he would likely want to resolve the issue. He could also become determined to prove himself to Sakura, that whatever she thinks he has done wrong, he is actually not that person.
In this kind of fanfiction, Naruto might be really frustrated with his friends or the villagers. Maybe he's tired of being misunderstood or underestimated. He could start by calling out their false assumptions about him. For example, if people keep thinking he's just a troublemaker because of the Nine - Tails inside him, he would angrily tell them how he's been trying hard to protect the village all along.
Their relationship could turn into a more mentor - like one. Sakura, with her medical and ninja knowledge, could start to teach Naruto things he didn't know. And Naruto, with his unique jutsu and experiences, could also mentor Sakura in different ways. This new dynamic changes the way they interact with each other and how they view their respective roles in the fanfiction.