For me, it's not that I'm in a Bah-humbug mood, its more that "Holy Crap, its already christmas and I didn't even realize it"
And then the added stress of finding appropiate gifts. I like giving thoughtful gifts, but if I remember too late, then I'm just boned. I'd rather give better gifts randomly throughout the year, rather then mediocre gifts specifically this weekend.
I was talking with my sister about this too. I wonder if that means she didn't get me anything good this year