Season 6 or later - Someone is being an asshole to Sam. Or maybe there are a lot of assholes around. Nothing personel, maybe just someone having a bad day and taking their mood out on Sam. Or someone not giving Sam the help he needs because it would be too much of a bother. Or something else that can happen anytime, anywhere, to anyone.
And Dean just snaps, and starts to randomly tell those people what Sam has done for them when he defeated Lucifer, and that they fucking owe him. He's pissed. Everyone else probably thinks he's a raving lunatic.
But a very scary one.