For me, I started because someone liked my short story and said he'd buy my book if I ever wrote one. Although I didn't start right away, I started 8 months later, and after that I liked writing as my motivation for success.
How about you guys? Did you start for a specific reason? Did you want to try something new?
My mother tells me I was "writing stories" when I was five y/o, so I don't know what my original motivation was. Wrote furiously all through college, then had a crisis of faith (in myself and my talent) and I just ...
stopped. For thirty years, to be precise. Then one day, about 4 years ago, I felt some logjam in my brain suddenly give way (and it did feel like an actual,
physical sensation of "something just gave way") and suddenly I started writing furiously again. I had to; the ideas were pouring out in a torrent, and this unruly horde of ideas made it clear that if I didn't get them down on paper, they would join forces and drive me mad. No, seriously.

The choice was either to write like crazy, or shoot myself in the head. I opted to write like crazy.