Two primary factors:
1) Weight loss/fitness. Loss of 15 lbs this season's commuting season (I choose not to commute year 'round), during period of increased beer and bbq consumption. Similar loss last year, for a total change from 220 down to 185 over two years. Reduction in resting HR, and increased cardiovascular endurance.
2) Elimination of feelings of "road stress" experienced driving, especially when driving home, and/or elimination of "bus dread" experienced when taking public transit home. The distance I travel, combined with the traffic during evening rush hour makes my bike commute take about the same time as driving or taking public transport, but I enjoy the trip, instead of dreading it.
Financial factors don't affect me. I still need (choose) to insure and maintain a car. I have two children that need to be transported around the city. I take trips to Costco. Any savings not buying fuel for commuting don't noticibly affect my quality of life.
Environmental benefits don't really influence my decision either. I can't feel too proud of my carbon footprint reduction, as I use A/C at home, consume high "food miles" food, etc.
On a negative side, I've had one close call with a car (had to bail), resulting in a couple of cuts and abrasions, and a couple of self-infliced crashes (one falling over in the clips and one taking a header due to getting stuck in a streetcar track) resulting in minor injuries (road rash, cuts, bruises including one to the ego). All three cases made me reconsider my choice to commute via bicycle. The near collision happened while riding on a mixed-use trail - I could not have taken a "more safe" route that day.