Summary: Because, Sasuke needed to let it out, and Sakura was there to support him.
Sakura often caught him bobbing his head when there was a catchy song on the radio. It was subtle—the way Uchiha Sasuke usually is, with most things. And whenever he sensed someone was looking, he would try to cover it up; try to make it seem like he was just stretching his neck or looking for something that both of them knew was not there.
So one day, she connected her laptop with the surround sound system while he was in the bedroom, and clicked on the trendiest, catchiest song she had. The speakers blasted and suddenly, every nook and cranny of the house was reverberating with music. Naturally, Sasuke wandered out to see why in the world was Sakura making their house quake on its foundations.
For a moment he could only stare in bewilderment as she swayed around to the music. When she saw him, she beckoned him to join. But because he was Sasuke and his dignity had roots stronger than the most ancient tree in history, he stubbornly shook his head. And then, unbidden, his foot started tapping to the beat and Sakura beckoned to him again. Once more, he stopped himself, and then she was there, catching his hands in hers, linking their fingers together and moving her shoulders to the music while leading him to the center of the room and suddenly he was dancing with her; his shoulders keeping up with the beat, his arms moving of their own accord and Sakura beaming at him proudly. Both of them "jammed" to the song—as Sakura put it—and Sasuke was taken aback by how much he enjoyed it.
Later, when it was over, he could barely meet her eyes from embarrassment as she laughed and wrapped her arms around him. But he was pretty sure he wouldn't mind doing it again.