Tons of stress, realizing the year is coming to an end, realizing things didn't turn out the way you were hoping, or they turned out like you feared they might...just plain ol' seasonal depression combined with this expectation that you're SUPPOSED to be happy and cheery, blah blah blah, and you're not, which makes it worse.
Personally, I hate Christmas. It always throws me out of whack and I get very pissy and depressed and having to be around family just makes it worse. This time of year is one I just want to hibernate through anyway, it's too dark, it's cold and there's still months of the same ahead.