Well that was certainly unexpected. Dean had caught a glimpse of Sam as the door opened, with a tired Sam and then the door...slam! In his face. Dean could understand Sam's reaction, but really, the last thing he expected was a door being slammed so nicely in his face. He expected some freaking out, some blubbering on Sam's behalf, the usual Sammy routine. So it was safe to say that Dean was stunned.
Groaning and rubbing his eyes with one hand, Dean knocked again. 'Sammy, open up.'
He really, really didn't want to go get his lock picking kit and pick Sam's apartment door open. If it came to that, he would do so, but for now, he'd try the nice way.