Agreed about the randomness of creativity, Dave. It's all around us, all inside us, in a chaotic state. But what I believe happens when we are miserable is that we are (over-)sensitised, and this sensitivity to events and states of the world around us provides a channel for the creative ideas to focus and come out through.

It doesn't have to be misery. Any other state of mind that creates strong sensitivity/vulnerability will do. Very intense happiness, for example. A normal happy state is not enough, however - not intense enough.