To the people who know you well enough to deserve a real answer, I would put it simply: I hope to lead a meaningful life, and this is what is meaningful to me. If you want to explain it in more quantitative terms, you might mention that we spend the majority of our lives at work, so it makes sense to pursue a job that you find challenging and more than just a source of income or stability. More recently I'd be inclined to say this is the only rat race I can stand...
For acquaintances, I tend to laugh it off with a self-deprecating joke—y'know, how you've always wanted a career with low pay, little recognition, and no demand? That kind of thing.