Why Do People Enjoy Kissing?

I think I know the reasons people enjoy sex -- both the proximate reason and the likely evolutionary reason (though I realize that the question there is quite complex, especially when it comes to the female orgasm).

But why do people enjoy kissing (I speak here of serious kissing on the lips, not just pecks on the cheek)? Is it purely social convention, or is it biological? Some people say that it's because there are lots of nerve endings in the lips, just as there are in the genitals; but I don't think that can be quite right -- one piece of evidence is that self-stimulation of the genitals is pleasant, but you never hear of people touching their own lips for sexual gratification.

If anyone actually knows the answer to this, I'd love to hear it.

(This post is prompted by Amber Taylor's Kissing Day post; thanks to InstaPundit for the link.)