When you're writing in addition to a day job and a ton of other commitments, writing becomes a chore to do at the end of the day when you're at your most tired and unenthused. The joy of doing Nanowrimo is that you drag yourself through it, no matter how you feel. You put those words down on paper (or screen) when, on any other day, you'd simply say "sod it" and go to sleep.
And then something magical happens. One day as you're writing you get swept up in the words. You feel what your character feels. Your heart beats a little faster. You want to cry, or laugh.
That happened to me today. I'm excited again. Tired, but excited.