"I'm not in the mood to write."
Of all the arrogant and ignorant excuses for not writing, this one is the worst.
Why is it arrogant and ignorant? Because you are presuming what kind of writing mood you're in before you can be in a position to know.
Here's a thought. What if you think your mental state isn't conducive to writing, but it really is?
I've begun writing sessions with an angry and bitter mind, thinking there's no way I'll be able to write. Yet within seconds of sitting down, I'm pouring myself out on to the keyboard. I've also begun writing sessions in what I thought was the perfect mental state to write, and found myself sitting there with no ideas and nothing to show for it.
The point is you usually won't know in advance whether you're in the mood to write. Worse, it's presumptuous and arrogant to think you can.
Just sit down to write. If it's doesn't work after a few minutes, that's okay. Give it just a few minutes more, and then permit yourself to quit. Don't miss out on a productive writing session because of your presumptions.