Piper wrote a blog about his 2010 writing leave. I thought his last reason for why he writes was particularly cool — and down to earth.
Finally, there is an inner impulse that I cannot explain that drives me to write. I would write if there were no possibility of publication. I have hundreds of pages that no one has ever seen but me, and it would not matter ultimately if they were destroyed. I wrote them not to be published but because there is an impulse from within.