Actually, in my case it was the other way 'round: I was sitting in a crowded bus, stuck in traffic, in a hot summer morning, figuring out how much time I'd spend running, skiing or walking the same distance... none of those were practical, but how about cycling. Zing! A few days later I bought a commuting bike and some panniers. Haven't looked back since.
My biggest obstacle remains the winter weather (or the fact that I need to find a place to dry my riding gear after a wintery commute). I've yet to commute throughout the year, but I'll get there one day.