The Possible Origins of Kissing
Despite the fact that while you're doing it, it seems perfectly natural, kissing is actually a learned behavior. The first documented kissing behavior appears in India and Europe around 2000 BC; faces were brought together, noses and faces rubbing together in a gesture of union. The practitioners of this in India believed that the exhaled breath of their partner was part of their soul. By being close enough to share breath, they were intimately intermingling their souls.
Around 1000 BC, an Indian text was written which contained references to kissing as we know now being used to express affection between lovers. Later, around the 6th century AD, the Kama Sutra documented techniques of lip kissing fully.
The Greeks were probably the first Europeans to take up the practice, and it spread throughout the empire. It survived through the Middle Ages, where the Church decided that kissing "in reverence of God" was okay, but kissing for pleasure was a venal sin and kissing with intent to fornicate was a mortal sin.
So, if It's a Purely Learned Social Construction, Why Do We Dig It So Much?
There are a few theories. The first fits with the assumption that a lot of behavior engaged in while in love is regression to childhood behavior in a subconscious attempt to appear youthful to your mate. Many cultures engage in premastication of food for children; mothers will chew a tough piece of food unto a paste and then place it in their child's mouth in a kiss-like action. In Central Europe, there exists a tradition where a man will place a piece of tobacco between his front teeth, then invite a woman to take it from him by mouth in a symbol of love. The practice can be traced to before the introduction of tobacco in the 17th century, replacing the plug of tobacco with pitch or resin.
The catch with this theory is that premastication is very widespread in human cultures, while kissing is present in few cultures which have not had contact with western culture. What I think is that it's a learned behavior which happens to turn on a bunch of instinctive stuff in our monkey brains. When kissing you are so close you can very carefully smell them, which helps us decide if this person is healthy and expresses the right genes to ensure future familial happiness. Lovers tasting each other's saliva get an even more detailed view of their significant other's health and genotype.
Interesting /msg I just recieved: "(r) rp says re Kiss: perhaps it may interest you to know that my Congolese girlfriend doesn't want to kiss, she thinks it's one of those silly things Mindele do."