Sasuke believes that he feels things for a reason. The most painful feeling he had ever experienced was after he came back to Konoha.
He avoided all known chakra, where possible. Sasuke knew it was inevitable that he would slip up and he would find himself confronted with a certain blonde, but he tried his hardest. He didn't know he would be greeted with a fist to his face, or that he would not make any effort to stop it. That wasn't even the most painful experience. The pain came shortly after while looking into familiar, blue eyes. Reflected back at him was the hurt, sorrow, and anger of what he had done to the person behind them.
Sasuke does not believe in second chances, but he knows that he is lucky someone else does. Sasuke knows that trust is slow to earn back, and he accepts this.
The time it took for Naruto to trust him again was monumental. Sakura, who had known Sasuke (although he is reluctant to admit it) before he left, frequently commented on his determination. Sakura admitted that she would have given up; she didn't have the motivation for such a task. Sasuke wondered what his motivation was, but decided that it didn't matter. The end justified the means, and the end was just something to work towards. Sasuke needed goals, and without them he was useless. What did it matter what his motivation was as long as he attained the end? Did it really matter what the end was anyway?
Sasuke tells himself that he is content, but he wonders what happiness is. One night, while star-gazing on a rooftop in the Uchiha complex, he admits this to Naruto.
He is quieter than Sasuke has ever seen him, and Naruto says that he is happy Sasuke is finally back. Every fabric of his being supports the sincerity in his words. Sasuke believes him, but something hurts inside. It feels like millions of things have taken flight inside his stomach, each looking for its own unique way out – pressing, ripping, tearing. He doesn't feel that it has heightened to its peak until after he realizes Naruto's body is on top of him, embracing him. Sasuke holds him. Naruto says that he missed him, and Sasuke nods. He believes that Naruto understands: Sasuke feels the same. Sasuke thinks he knows what his motivation was all along, and he is suddenly aware that Sakura knew before he did. Surely, if he didn't know, they wouldn't be gripping each other with the passion they had now.
Sasuke remembers every moment from that night with a genuine smile etched onto his face. He wonders if this is happiness. Sasuke thinks that if happiness is anything but this, it isn't worth it.