"You already are good for me," Sam reminds him, taking another step closer and wrapping his other hand around his cock, giving himself a stroke. Just enough so that a bead of pre-come comes out, just on the head. "Open your mouth, Cas. You're gonna take me all the way down before I come for you."

It gave Castiel something to work toward, something that Sam coming would be reward for. Not that Sam would deny him his orgasm if he didn't manage, but he knew Castiel could do it.

He takes the last step forward, the head of his cock brushing against Castiel's lips.