The simple answer is that these days I go where the muse takes me. I'm basically on a year-long sabbatical after leaving Google this summer, where I'm not particularly worrying about making money. I did my PhD on tools for interactive curve design, and I was frustrated while I was at Google because IP issues made working on curves complicated. So when I had some new ideas, it was like the dam bursting, and I've been enjoying lots of motivation and creativity. I started the codebase for kurbo on Christmas and wrote 1300 lines of code that day. I'll keep doing it while it's fun.

Clearly, people like reading about it (it's also really easy to miss that there's a surprisingly high number of Bezier-related posts getting on the front page year round. They usually get there and then fall off again in 8-12 hours =)