Funny thread.
My answer to the question is - becuase I enjoy the feeling, the release, it helps me sleep and it's a good chance to think about women who have been important in my life, those times I've really enjoyed and the fantasy of those times I'd like to enjoy.
As I am "dating" sporadically, apart from the obvious reason, I find it gives some kind of guage to how I feel about someone I might want to see again. If I don't think about them then, it's probably best to remain "just friends"!
While I was sitting accounting exams, I discussed with a male and a female colleague how the boredom of the textbooks often promted us to take matters into our own hands and have a few minutes of pleasure. All 3 of us agreed that boredom is a big turn-on!
KP