One of the reasons that gaming is so pleasurable.
I also feel it when I get particularly good at a task at work. My current favorite work-flow state is tracing out our electrical grid on maps for special projects. It wasn't fun at first, but now I find myself looking for excuses to bust out my ruler.
I also feel it in public speaking. I'll start off a little nervous, and then after a minute or so, I'm just not there anymore, the words tumbling out, exactly the way they're supposed to.