Why write about sad stuff?

A good friend read an early version of Infinite Sum and asked why I was still writing about the sad stuff. For those who haven't read Divide by Zero, it tells a story of how abuse affects a small-town community. Infinite Sum makes it all that bit more personal by looking at how it affected an individual within that community. So yea, it's sad stuff. And my reviews in this post will all be related to sad stuff of one sort or another. So... why write sad stuff, and why do people read sad stuff?...