I had the same thing; my first impression was that Sam would be the one everyone decided was the bottom, because he was younger, more likely to get visibly upset an weepy, and more likely to do what Dean said than the other way around.

And it turned out to be completely the opposite. Which is kind of cool, because I like it when the guy who reads less like a weepy-uke cliche to me bottoms (I'm weird that way), but kind of weird it the way 'Fandom has decided that this one guy is always the bottom' tends to be.