How else would you interpret it.
SAM: Dean, it’s Valentine’s day. Your favorite holiday, remember? I mean, what do you always call it— Uh, unattached drifter Christmas?
DEAN: Oh, yeah. Well… be that as it may…I don’t know. Guess I’m not feeling it this year.
SAM: So you’re not into bars full of lonely women?
DEAN: Nah, I guess not.
It’s because of Castiel isn’t it. Isn’t it?! I think so. Yes. It’s because of him.
I mean… is this not obvious to everyone else?