It's quite a physical sensation, and can be influnced by lots of sensations. Recently I was driving home from work. I was wearing new gloves, and didn't have them tight enough, so my fingers felt clumsy, I had just picked up the bike from the garage, so it felt like things were off, I was just back from 1 week with Tom, so I had a weeks worth of clothes and stuff weighing me down. It just didn't feel right. I had left work with a decent attitude and riding the bike home totally soured me. Nothing felt right.
When I got home, I dumped all the stuff in my bag, leaving just my laptop, and put on my old gloves. I rode down to the hackerspace. This ride (5 minutes later) was completely different. I was much more free, I was in control, I was in a good mood, I was flying. It was a wonderful feeling and totally improved my mood for the rest of the evening.