the power of self locomotion...
Adam Driscoll has a nice post on his personal blog talking about how biking to work has made a huge difference in his happiness. It's a great post explaining the tradeoffs and benefits.
I too find that when I walk to work or get around I feel much more alive, because I am in the act of living, not the act of being carried around. You have to be more aware in an odd way when you are walking and riding than if you are being driven around. And being more aware is being more alive.