I got started through my own studio I co-founded back in 2009/2010. As we got going, it became quickly clear that I was by no means the best programmer in our small group of two. In fact, I'm lucky to be able to read StackOverflow or a forum like this and follow the Java/Obj-C enough to get what I need done. Programming was just never my forte, unfortunately.
However as we got going, I started to learn about the field of interface and interaction design, and it intrigued me so much. I started to learn that designing an interface wasn't just about creating pretty pixels, but instead about figuring out how and why buttons/text/views should be placed on screen. I've always been a science/math minded person, and it was extremely interesting to learn that there are great principles in play as to why operating systems have design guidelines, and why content should be displayed to a user in a specific way.
The book that kinda inspired me into writing on the field was The Humane Interface by Jef Raskin (http://en.wikipedia.org/wiki/The_Humane_Interface
), which was also published by Addison Wesley (imagine my excitement to know we'd have books through the same publisher). For those who don't know, Raskin was considered to be the father of the original Macintosh before Jobs kicked him off of the project. He then went on to work on the Canon Cat, and many of his ideas on interface and interaction have become common place in computing today.
Jef was really my biggest inspiration as to why I should write about this stuff. My background is in economics, everything computing/design has been self-taught. So I had the fortunate circumstance of having gone through a college path where I had to frequently write and explain complex systems, so the idea of writing technical books on interface seemed fun.
But as for why I did interaction/UI work, I've just found it as fun. I've always been into computers, so this has been my way to make a career out of them!