Really I just don't like to work. (For someone else, on something that isn't interesting.) I would like to believe that the hours I spend each day studying, playing music and exercising count as "equity", but I could just as easily see it leading to nothing career-wise. The funny part is, I'm happier than I have ever been!
BTW my 2009 AGI was $22k, and I live in the Bay Area. It's doable.
It was doable primarily because I always lived with people who had rent controlled apartments or houses they'd lived in for some time. The cheapest rent I paid--and where I lived longest--was $550/mo. My girlfriend lived with me there and we split it 50/50.
Other things contributing to low living expenses: no car (huge savings); no TV; aggressive (but perfectly legal) tax deductions; choosing fun activities over costly purchases; freegan friends who gave us loads of food; and, of course, two buck chuck...
It's certainly doable.
A downside of living this way is, obviously it would never work with a family. So if/when it comes time for me to do that, I will probably have to grow up and get a Real Job.
Until that point though, this is great! I look at all these people, friends, working 60 hrs a week and completely missing out on their 20s and 30s, and I feel like I've stumbled on to a well-kept secret.