One reason could be that Naruto has been through so much himself and he might think rookies are naive. He's seen the harshness of the ninja world and doesn't want to coddle them. For example, he had to grow up without his parents and faced many life - threatening situations early on.
Maybe Naruto is cold to rookies because he's trying to toughen them up. He might believe that being too friendly will make them complacent. In his own journey, he had tough mentors like Kakashi who didn't go easy on him, and he could be following that same principle.
It could also be that Naruto is so focused on his own important missions and responsibilities that he doesn't have time to be warm to rookies. He has to protect the village and deal with powerful enemies, and he sees rookies as a distraction from his main goals at first.
It creates conflict. The rookies might be initially discouraged or resentful of Naruto's coldness. This can lead to them trying harder to prove themselves, which drives the plot forward as they strive to gain Naruto's respect.
One possible reason could be that Naruto has his own unrequited love for someone else and he is trying to distance himself from Sakura to avoid leading her on. For example, if he has deep feelings for Hinata, he might be cold towards Sakura to make his own feelings clear.
I'm not sure specifically as I haven't read that particular fanfic. But it might be about Naruto getting angry at the rookie ninjas for some reason, perhaps they made a mistake during a mission or did something that goes against his values.
One reason could be that Naruto is focused on his own goals like becoming Hokage and he might see Sakura's feelings or her actions as distractions. He has a lot on his plate and may not have the time or energy to deal with her in a warm way.
Perhaps he reveals his dreams for the future of the Hidden Leaf Village that are more personal and not part of his official Hokage - related goals. For example, he might talk about his hopes for how the different ninja clans interact and co - exist in a more harmonious way, something that he has been thinking about but hasn't shared with the group yet.
Obviously Naruto is one of the main characters since the title mentions him. And the rookies are also main characters, though we don't know exactly which rookies without reading it. It could be the ones from his own generation like Sakura, Sasuke, etc., or it could be new rookies that Naruto is mentoring.
Another reason might be that Naruto has been influenced by external forces. Maybe he has made some bad friends or been involved in some situations that made him think being cold to his family was the right thing. He could have been brainwashed into believing that his family was holding him back from achieving something greater.
She might be confused at first. Sakura is used to Naruto being friendly and now his coldness would be a big change. She may start to question what she did wrong.
They would probably be shocked. Since Naruto is such an important figure to them, any big secret would likely cause a stir. For example, if it's about his Nine - Tails secret, they might be initially stunned as they realize how much he has been carrying alone.
'Cicada and Robin' was a classical Chinese text that described a conversation between a cicada and a Robin. The cicada said to the bird,"Although you can imitate human speech, what you say is not what you want to say, unlike my own voice that can express my feelings." The red-robed bird felt ashamed and stopped imitating human speech. This story implied that people should have their own independent thoughts and expressions.