Probably no good reason really...
Truth of the matter is, I enjoy running. Nine years in the US Navy and I never liked it a bit. Six years after I left the service (2007), I picked it up and just kept going.
My job has a fairly odd schedule. Fortunately, we are off a fair amount of time throughout the year. But, when I am working, it's either 10 hour or 12 hour (more of the 12 hour) shifts. I think that my main reason for tweeting my runs (which then feed to Facebook) is to show that it's possible to get out and be active. I don't have quite the amount of family commitments that a lot of folks have (kids/kid activities), but movement is still possible. Get out and enjoy the fresh air!