Maybe he's just showing off because Dean's secretly watching him but of course Sam heard Dean's tiny moan it's going to be a very quick work out and just figured he'll strip his shirt off and thats it.

I hope there is PLENTY on the gag reel of this scene. You know Jensen probably kept telling the director that something just doesn't look right so they had to redo it, over and over again with a new 'problem' each time.