He was mature when it mattered. Besides, Dean was a big kid at heart. Dean was the one with the childish sense of humour. But it was Sam's official job as a baby brother to do everything in his power to wind Dean up. It was the rules.
Sam wiggled his eyebrows at Dean. "Maybe I want you to put me to sleep another way!" It hadn't been his intention when he asked Dean to stay, but being fucked into the mattress so hard he couldn't think about anything else definitely seemed like a good idea to him. Hell, even being punched unconscious would work.