I have the same reason as many others.

I started bike commuting two years ago, a few years before the birth of my first child.
I hadn't done any exercise for at least a decade, and that coupled with a sedentary job meant I was very unfit.

I needed to find a way to keep fit that was reasonably cheap, didn't take too much time and didn't involve going to a gym. Cycling to work was the obvious answer, although the 36 mile RT takes 90 minutes longer than driving it means I get 7.5 hours of exercise a week without affecting my weekend time with the family.

I also find driving to work quite stressful. Cycling to work is always better, even when it's raining and windy. On a clear still sunny day it is fantastic.