It doesn't matter what our reasons are. We could be writing a novel in a month just to know if we can, to see if we want to be writers, because we are bored, because we want an excuse to not talk to anyone at Thanksgiving. Maybe we want to try that novel idea we've known forever, or the one we thought of today. Maybe we just want a challenge.
You're going to be asked “why” a lot this month, and by all kinds of people; the family you're avoiding, your friends who miss you, intrusive strangers, your boss, and by your characters themselves (that is, if you're the kind who gets caught off guard by your characters). You will even ask yourself, at a certain point.*
There is no wrong answer, but it is important to know. Petty or deep, meaningful or flippant, keeping that reason in our minds helps us get where we want to go. It's one thing to drive across the country, it's another to drive across the country to see friends we haven't seen in ten years, or because we never drove anywhere alone before; the journey is more important when it has meaning, and more likely to be completed.
If we know why we are writing, we will keep writing.
Put your reason on a sticky note and stick it on your keyboard, or on the window you look out each morning or your mirror (but I hate putting things on my mirror), and keep in mind why you're spending most of your time writing or thinking about writing instead of doing normal-life-things. Like remembering to mascara on both eyes, or carrying on a full conversation without taking notes.
So now, why are you doing NaNoWriMo?
*Knowing what to tell them saves us from that small and embarrassing existential crisis after someone asks us a question we don't know the answer to, but probably should.