Great thread. To quote Robert Pirsig (who was writing about riding a motorcycle, but it's apropos for bicyclists as well): "In a car you're always in a compartment, and because you're used to it you don't realize that through that car window everything you see is just more TV. You're a passive observer and it is all moving by you boringly in a frame. On a cycle the frame is gone. You're completely in contact with it all. You're in the scene, not just watching it anymore, and the sense of presence is overwhelming."
Being in the scene on my way to work makes me happy: it also tends to reverberate throughout my day, and thus I'm more "in the scene" while working.