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An article I saw on the flight to Baltimore, MD -
http://www.abc.net.au/catalyst/stories/s498838.htm

(excerpt)
The long answer is that it is tied up in pheromones. When we are close to another's face we get an olfactory cocktail that tells us a lot about each other. Women sniff out signals to tell if a man has the proper diversity of immunology to create a good child. (just as well as you pass on around 250 colonies of bacteria with every smooch) Men pick up on similar signals of fertility and strength, (and how expensive her perfume is) to help decide if she is the one for him. 

The short answer, of course, is because it feels good. ...


See also "Fun Facts About Kissing":
http://hicards.com/valentine/vkiss.html 

(excerpt)
The average person spends two weeks of their life kissing.