So many things. It's faster than the bus, and the money I saved in the first year paid for my bike. Because of the riding, days when I'm not commuting now I explore and have seen parts of Chicago I never drove to, found some nice neighborhoods, great parks, etc.
My route to work is beautiful (for a city) and peaceful, the first half through a lakefront park but on quiet roads so I don't fight either car or bike/ped traffic, just herds of geese. I can zone out, get some exercise, wake up in the morning or decompress on the way home. I can take the long way if I want and see something new.
I love the act of riding, even in my Frogg Toggs in the rain, and being originally from a lot farther north the winter cold doesn't bother me, either. On days when I feel like crap and have to force myself out on the bike, within a mile I feel great! I've never felt better since riding every day.