Not being able to ride much recently has made me ponder this question a little more.

The aerobic high I get after just an hour in the saddle usually lasts six hours after the ride. It's more relaxing than a stiff drink and thus, I find myself abstaining.

I've tried many other forms of exercise: running, rowing, treadmill, elliptical, but nothing compares. They're all too boring because you're either moving too slow or not moving at all. So I think the second part of my answer has to do with how fast the scenery changes, and how many things you have to focus on to remain safe. I am NEVER bored on a bike. I have a touch of A.D.D. and I've read that some people call cycling their Ritalin. So that's probably true for me as well.