James Somers in More People Should Write:
You should write because when you know that you’re going to write, it changes the way you live. […] When I have a piece of writing in mind, what I have, in fact, is a mental bucket: an attractor for and generator of thought. It’s like a thematic gravity well, a magnet for what would otherwise be a mess of iron filings. I’ll read books differently and listen differently in conversations. In particular I’ll remember everything better; everything will mean more to me.
He also repeats an argument I’ve made to many friends:
I have a selfish reason for my demand: I have a lot of friends who are thoughtful, but keep their thoughts to themselves. I imagine finding notebooks under their bed, tens of composition books packed with little print. I think about what sort of a treasure that would be.