The New York Times has posted a long excellent interview with Joao Silva and Greg Marinovich on the Inner Life of War Photographers. It's a highly recommended read.
Keller: "This is a question that I know you guys wrestled with a lot for the book, and it’s still something that puzzles people, which is: why do people do this?"
Marinovich: "It puzzles my wife."
Silva: "The reason why I continue doing this and writing the book — my focus has always been to be on the edge of history. I’ve always wanted to show the reality of war to those who are fortunate enough not to live in a war zone. That’s pretty much my standard answer. I think if you go deep down, it’s a lot more complex than that. That’s just how we rationalize this. But, certainly, being on the edge of history is real, you know? Those few moments of adrenaline, which are very short; those few bursts of activity. I think it’s complex. It’s not just one single answer as to why we do this kind of stuff.
We do it because it’s there, we do it because we can, and because there is a need for the world to know. And we can go out there and do it."