William S Burroughs said that he hadn't really thought something until it was written down. The act of writing completes what started in your head and in some instances can be an act of absolution. Or is it absolvement? In any case, there is a certain special kind of restlessness that attacks your limbs if the writing is neglected.
I had a blog post that wouldn't quit. The subject had seemed exhausted, the writing complete. A few days later it hit me that the subject of the post was a certain kind of person whom I hadn't represented correctly. I had failed to recognize his qualities as a bastard. This could not be. I could not rest without full disclosure.
The issue can be phrasing, or a point of grammatical exactitude; it can be that what's written doesn't fully convey what I'm thinking. This last is too often the case, as words express only partially what is thought even in the best scenario. Understanding doesn't quell the restless quill, and it can even make things worse: the runaway tiger of complete expression always has a greased tail.
Rainer Maria Rilke advises that writing is do or die, and I take odd comfort realizing that finally choice is not part of the bargain:
No one can advise or help you — no one. There is only one thing you should do. Go into yourself. Find out the reason that commands you to write; see whether it has spread its roots into the very depths of your heart; confess to yourself whether you would have to die if you were forbidden to write.