I just wanted to be cool. I wish I had a better reason like most of the ones given above, but 15 year old me just wanted to be cool; my mates had a fledgeling band with three guitarists and a drummer, and I figured that the way in was to play (read "buy a ") bass and learn on the job. In reality I didn't know what a bass was for or how it was used, so I found it all a bit unfulfilling and changed to rhythm guitar by the time I was 20. Fast forward to a couple of years back when I decided that I wanted to play bass again, had a bit more experience and was more willing to learn my place, and the circle was complete. I'm much more at home on a bass now that I was as a teenager, and I'd say that although it's not a high bar I'm better on bass than I ever was on six strings.
Whether I made it to being cool, though.....