With writing, it comes from the thrill of hearing and (I hope) telling an exciting story.
With drawing, it's a bit different. I try to explain to kids that my brain looks for patterns everywhere - in wood grain, in the curve of a dark hair on white tile flooring, in shadows in clothing.
I was at Hillcrest School this week, and saw a perfect example of what I'm talking about. This basketball hoop in the school gym is clearly a cat.
Here's what my brain sees... and then I just have to draw that face.
And the bars on the top could even be the antennae on an alien kitty!
It's not the only place I find inspiration, but it does illustrate the maxim that inspiration often finds you.