Taking inspiration from the always inspirational aliera9916, I have my own quotage to share:
“Writing a book is a long, exhausting struggle, like a long bout of some painful illness. One would never undertake such a thing if one were not driven by some demon whom one can neither resist nor understand.” ~ George Orwell
I have long thought of the main character of my novel as my muse, and probably it is the case that I have multiple muses, and they are always my characters. But to think of the urge to write as a demon is an interesting stroke of insight. For what could “possess” me to sit at my computer for hours a day, pounding on they keyboard like my little icon demonstrates? I always thought it was my characters, begging to have their stories told, giving me no peace until their voices were heard.
But what but a demon could force you to work for not months but years, neglecting the rest of life’s other pains and pleasures?
This demon is an incessant need to put words on a page, on a screen, spewing them out like a scream, like vomit, like one of those really, really long urinations you have after you’ve been forced to “hold it” for hours after drinking way too much coffee.
Oh, ho, get your metaphors and similes here.
Author Nancy E. Shaffer 
I may not look at my books quite the same way now.
More seriously, if there’s one thing that LJ has done, it’s given me a much deeper respect for what writers go through in the creation of their work. It reminds of of that other quote about sitting in front of a blank sheet of paper sweating blood.
Writing metaphors
Well, my comments have to do with the project of writing a novel or any book-length project, which goes way beyond the act of writing itself, into plot planning and editing and the discipline of keeping to a regular schedule and so on for months and years.
It could be true of any huge, long-term project, from business to art to whatever.
I am familiar with the “sweating blood” quote, but I think any one who has faced the prospect of writing an assignment or a letter or whatever and has stared at a blank page has had that experience, not just folks who consider themselves “writers”.
But I’ll let you in on a little secret. For a writer, writing is one of life’s greatest pleasures. I didn’t drag in the sex and orgasm similes and metaphors, but hey, they fit, too. It’s like a little kid who can’t keep his/her hand off their private parts–you write ’cause you have to, and it feels good. And the next day, you write again.
Some days it comes out good, some days it comes out mediocre, some days you just can’t “get it up”. Sometimes you can go through a really long dry spell where you just ain’t getting any.
But you never can stop for good.