It developed from when we were animals and mothers would feed their young by putting food from their own mouthes into the mouths of the young. Feeding is obviously an experience associated with pleasure - the experience of pleasure when our mouth nuzzles another has remained.
What's the betting this answer causes a few people to over-think whether kissing is desirable?
Do people of all cultures kiss? I thought that not all cultures have kissing, meaning that it wouldn't necessarily be instinctual, just culturally tenacious.
Im not so sure about your theory WaldoMcFroog, mammals suckle their young until they can eat for themselves so there was no need to feed mouth to mouth like birds do...
fair point?