"Sam will keep. We're not done here yet," Lucifer says in the same mild tone as he stops petting Dean's hair in order to form a firm grip on it to tilt Dean's head to make him look at him. "I want your mouth again before I leave. I recommend you actually do your job this time unless you want to be gagged again and have an additional two months added to your silence."

Is it cruel to make Dean do this with the knowledge that Sam was right on the other side of that door? Probably, but it will help cement Lucifer's dominance in Dean's mind.