I think it must be some sort of mental disease. I remember a case on "To Catch a Predator" where the man they caught was already awaiting trial for being caught before -- on the same program!

When they get caught, these guys know their lives are over, and they often break down crying. They're soldiers, clergymen, teachers, elected officials; in other words, not the scum of society. They know they have everything to lose, and yet they keep doing it. You've got to be mentally ill to keep doing that to yourself.