If Sam's sounding like a whiny teenager then Dean's not doing much better, because at least Sam's argument has something like logic, however biased, behind it. Dean's just running on loyalty and duty which, fair enough, serve him pretty well as fuel 99% of the time. Here in that other 1%, though, they leave him sounding equally stupid, though in an 'old and dogmatic' sort of way, and while ordinarily he'd be flattered to think he was turning out even a little bit like Dad? Right now, not so much.
< At least I'm not just Dad's puppet
… totally the wrong response, yeah. Because crossing that line flips the switch from just standing and yelling worn-out platitudes at each other; for a moment it's like Dean's just going to laugh it off and turn away, except that anyone who knows him would never be fooled by the hissing smirk and the way he shakes his head and turns away. Then it's a second or two later and he's barrelled across the room and set to giving Sam a hasty and vigorous introduction to the stretch of wall next to the door.