Another block for a writer's motivation is discouragement.
And there are a lot of ways a writer can become discouraged. If you are a novelist in a boat similar to mine, you have a few books out, which don't seem to be going anywhere.
Things which discourage me:
Rejection by a publisher. I know. You have to expect this. You receive far more rejections than you do acceptances. So, if rejection is debilitating, success as a writer will be impossible. However, a little rejection, or a lot of it, can stall you and give you the opportunity to evaluate where you are and what you need to do to move forward. It can also be renewing.
Another thing that discourages me, and this may sound absurd, but when my editor cuts things I like. Or if she is less impressed with my work than I am. I realize that cutting up my manuscript is what I pay her for and that if I don't agree with her, I can keep what I want. But, her purpose is to my my manuscript better, and while it sometimes hurts, taking her advice is, well, good advice.
This last week has been discouraging with my book cover for, "Shooting Stars". It had nothing to do with the manuscript I have been editing, but every time I got the notice from Create Space that my book cover didn't fit their template and that parts would be cut off, I got more discouraged and ended up ruminating on what was going wrong, more than what I could be doing right.
However, after the fifth submission of my book cover, I got the notice that it was accepted. Hooray! Now, I can move ahead with a book launch later this month. Suddenly everything is bright and encouraging.