For me nothing feels better than knowing I've put in a good day's work doing what I love. Just at the moment my writing is going well and I'm at peace with the world. (Not always the case!)
Naturally I have to ask why this is happening. So I'll know for the next time I'm not feeling as good about writing. Is it the weather? The changing season? What's giving me this sense of satisfaction? What makes me jump out of bed in the morning and rush to the computer?
Basically it comes down to two things.
First, I'm making a bit of visible progress every day - a new scene, a bit of new dialogue, a new idea to explore further, etc.
And secondly I like what I'm coming up with. I like my story, I like the characters, I like the moral questions I've raised. I'm excited because I can see that the finished product might just be worth all the effort it's taking me to create.
That's it really. That's all I need to gain a sense of satisfaction, to keep me coming back to the page. Not the knowledge the writing is perfect but that I have the skills to fix what's wrong.
So believing is what yields the largest measure of that satisfaction. Believing I can meet the challenges of creating a work. Believing in my imagination to furnish ideas and my abilities as a writer to express them.
So a note-to-self for those times when the writing isn't going so well: There's no downside to believing in yourself. If you can't, pretend you're a writer who does.