Why? Lots of reasons. But I think right now, the biggest reason is that I've made myself accountable to someone. Or make that a few someones. So that's my wisdom for today: if you find yourself in a writing rut, make a goal and then make yourself accountable to someone. Choose wisely; make it someone you would never, ever want to disappoint. Someone like your CP, or your spouse, or your mentor. Assign yourself a reward for when you meet your goal. And then, when that fire is good and stoked, go after your goal.
For me, the secret is in the numbers. And it can't be someone who would love me anyway; it has to be writer-friends, whose professional opinion of me would be partly formed by whether or not I can achieve this goal. And sometimes, I don't even tell them. I just think to myself, so-and-so would expect me to be able to finish this by such-and-such a time.
How about you? Who are you accountable to?