So many great answers in here ... I could echo many of them.

It's fun. Beats driving. Saves wear and tear on my already old vehicle. Saves me money on gas. I'm healthier. It puts me in a good mood, before work, but especially after work. It also helps me wind down and deal with any stress from work. There's something very gratifying about knowing that not only CAN I do it, but that I choose to do it. And yeah, it's kind of cool to have co-workers notice and be a little surprised when they find out I do it and how far I ride. Had one guy see me in the parking garage on Tuesday, "Didn't ride your bike today?"

Plus it's a ready made way for me to get my miles in without giving up a lot of my time after work.