I believe NaNoWriMo could be called crack for writers.
It effects your relationships, makes your brain go a little crazy and after you stop, you suffer from withdrawal. This happened last year as well- once you get into the habit of writing everyday, you feel like you still should. Not a bad habit for someone hoping to be a professional writer, actually.
Last night I wrote a new scene with my characters that will appear in an eventual sequel. I think I feel it even more acutely this year because I adore my characters and plan on spending more time with them- as I said, sequels.
My friend and I did a novel swap, I'm currently reading hers. Tonight I experienced that awkward moment when you realize you're a side character in your friend's novel. That was new. Not terribly unpleasant.