Arashi here. This thought struck me while being dragged through a clothing store. Half an hour later, I convinced my mother to give me her memo pad.
Futility
Because Lee is smarter than most people give him credit for.
Lee isn't stupid.
He can calculate the amount of force needed to kick someone to the ceiling, and how fast he will need to be to send them spinning to their doom. He knows the trajectory needed to begin the Konoha Whirlwind if the opponent is twenty meters away on a fifteen degree incline. He knows exactly where and when and how to hit to crack a boulder in two with a single punch, without even pulling a muscle from the needed force. And he can tell, calculating variables and probabilities the entire time, how much unrestrained power he can exert without killing himself from the strain.
Butwhen Sakura turns away from him to stare after Sasuke's retreating back, he knows he can only sigh and watch her go.
Because Lee isn't stupid.