Honestly? For the first time I did, it was because both my brother and I were taking guitar lessons, and he was way better than me. One day, my teacher brought in his five-string Yamaha, and I picked it up and started playing some RHCP right off the bat. So my parents bought the bass off of him, since it was for sale anyways.
Yup, I'm one of those guys who sucked at guitar!
But, you know those after-school specials, where the popular kid loses a bet and has to take the ugly girl to prom, and then finds out that she has a great personality? Yep, that's pretty much what it was like. Didn't know what I was missing until I played.
Lessons ended shortly thereafter, the bass sat in a closet for three years. And then Guitar Hero 2 came out. or, at least, the Demo. And I was captivated by a song called YYZ, just the guitar line. A friend comes over to play co-op, he plays the guitar section, and I play the bass part, and I see that the bass has almost twice as many notes.
The rest, as they say, is history.
I really prefer the bass to the guitar, though. If someone asks why, honestly, I just find it a lot more tactile, but I love the boom. I guess that's why I would always watch the bass players in bands in my adolescent youth.

I still noodle around on the guitar a bit, but I love my bass so much more.
My brother is an awesome fingerstlye guitarist now, though. But, to each his own. We'll see what we can do when my fretless sixer arrives. It'll be kind of weird to hook up now that I'm shipping out to college, though.