Spite.
After my first evening class course, on an Apple II my brother looked at my work and said "Is that all?".
I had initially taken the course because I wanted an idea of how the video arcade machines worked, that I was spending an inordinate amount of time playing.
After the 'challenge' from my brother I delved deeper and became a developer for a living. After many years I lost interest in writing interfaces and pretty front screens and worked my way further 'behind the scenes'.
I do it because I enjoy the intellectual challenge of it (usually) and it isn't the most dangerous profession I could have chosen.
I now work on large databases using a variety of techniques. I've worked on stock-control systems, medical, sales, housing and a plethora of others.

My brother worked as an animator on Lexx, the Wallace and Gromit games, some Zoo simulator (if I recall correctly) and is now lecturing and taking a Phd in running around with ping-pong balls glued to himself.
Death is too good for him.