Naruto is the creation of Kishimoto Masashi, I own none of it.
Reason
Naruto thinks that he can help. He thinks that there is nothing that cannot be achieved through force of will alone. He thinks that Sasuke has not really betrayed Konoha. He thinks this because if Sasuke has betrayed Konoha then Sasuke has betrayed him, and Naruto will not contemplate being betrayed after finally being accepted. He will not think that maybe there's a chance that Sasuke will not come back, because that would mean he failed and Sasuke does not think as highly of him as he thinks of Sasuke. Naruto thinks that the reason he will not give up on Sasuke is because of Sakura, he made a promise to Sakura and he always keeps his promises. Naruto thinks he can help, he thinks he can get Sasuke back and they will go back to their three-man team, he will kick Sasuke's ass because he's so much stronger and no matter how much Sasuke calls him dead-last he'll prove that he isn't, he can bring Sasuke back because he is Naruto and he can't, won't, can't think of what it might come down to if Sasuke is really serious and won't come back, but he shouldn't even be thinking this because of course he'll get Sasuke back, he made a promise to Sakura after all.
Naruto visits Shikamaru to tell him he'll be leaving to train, and Shikamaru tells him to kill Sasuke. Naruto replies that he shouldn't say that, Sasuke is still part of the Leaf, Naruto won't allow him to go the way of his brother because they have a history and he made a promise to Sakura, after all. Shikamaru just shakes his head and thinks of Chouji, Neji, and Kiba. He won't say anything now, it's too troublesome to part on bad terms, but he knows Sasuke is no longer part of the Leaf. Sasuke has betrayed them in his mind and Shikamaru knows that that is the only place that matters.
Naruto says he needs to hurry, the ero-sennin is waiting and he doesn't want to waste time, can Shikamaru say goodbye to those not conscious for him? Shikamaru nods, and Naruto hurries off to find Sakura. Shikamaru watches him run off, and resolves that he will not fail on his next mission.
Naruto reassures Sakura that everything will be alright, he's going off to train and he'll definitely get Sasuke back, he won't break his promise to her. Sakura smiles through her tears and tells him to be careful. Despite everything, Sakura still cares about Sasuke and hopes Naruto will be successful, even though deep in her heart something young and innocent has died and she knows that nothing will ever be the same. She thought she loved him, she thought that one day there might be a chance he would love her back, and he betrayed her unstinting trust in him. Even if he didn't love her she had always known that he would always be there for her, he would always be there to protect her and he would never betray her, never betray Naruto, never betray Kakashi-sensei, never betray Konoha. She had known this, believed this deep in the core of her heart, so that even now she sometimes thought that it never really happened, she had managed to stop him at the gates and he didn't leave. But he left, and so she smiles at Naruto and hopes for his success while knowing there can be no success, Sasuke's betrayal has cut too deep and it's too late to heal the wounds he left behind.
Naruto thinks he can help, he thinks that he can bring Sasuke back and everything will be better. He doesn't think of what will happen if he does bring Sasuke back, he won't can't won't think that Sasuke has already committed a betrayal too deep, that Konoha might not welcome him back and everything might not go back to how it was. Naruto knows that he can do anything, he knows that he will become the strongest fighter in the country one day, he knows that he will be Hokage, he knows that he will control the demon within him, he knows that he will bring Sasuke back. He thinks that that should be enough, that should be enough to make everything better, that should be enough to make Sasuke realize that power is not worth betrayal, Sasuke will come back and they'll go back to being eternal rivals and bringing him back should be enough but it's not. Naruto doesn't want to think about afterwards, though. Naruto wants to think that there is nothing he cannot do, but Naruto thinks treacherously that he could not defeat Sasuke and Sasuke might come back but Sasuke won't come back, it won't be the Sasuke he knew, it won't be Sasuke his best friend but Sasuke the betrayer who Naruto still thinks of his best friend because Naruto thinks he should be able to do anything.
Naruto thinks he will bring back Sasuke, come what may.
Naruto leaves.