There's apparently a evolutionary significance to kissing. Who knew? I'm an evolutionary biologist by background, and I didn't know. Me, I just thought it was kind of fun.
Wendy Hill, a neuroscientist at Lafayette College in Pennsylvania, published research a couple of years ago that indicates that there are changes in levels of hormones when couples kiss. Apparently, Hill's team paid heterosexual couples to kiss for fifteen minutes, and measured blood levels of various hormones before and after (and I can only imagine the lines of horny college guys waiting to sign up for this opportunity). (Source)
The results were intriguing. In particular, the hormone oxytocin seems to be affected by kissing. Oxytocin is one of the "feel-good hormones," and has been nicknamed the "cuddle hormone" because it is associated with the maternal instinct and caring for an infant, and the fact that its levels skyrocket in both genders immediately after orgasm. The research indicates that oxytocin levels spike in men during kissing, but they fall in women. This I find surprising, but I can't find anywhere that the researchers speculated as to why oxytocin falls in women after they kiss. This to me would seem to indicate that men feel better after kissing and women feel worse, which seems a little odd. Maybe it's because kissing makes men think about having an orgasm and makes women think about taking care of a infant.
In any case, it's interesting that 90% of human societies (according to the research study) "practice kissing." I don't know about the other 10%. Perhaps they rub foreheads together, or something. Perhaps they don't practice any more because they've figured out how to do it right. It's a mystery.
The other intriguing find of the study was that men prefer "sloppy kisses," whereas evidently women don't. The researchers explain this by positing that saliva contains trace amounts of testosterone, which is linked to increased sex drive in both genders, and swapping spit is a way of dialing up the response in your partner. So, I guess that sloppy kisses are just another human male equivalent of the peacock shakin' his tail feathers -- a chemical way of saying, "hey, baby." So, it falls in the same category as going to the gym to build up your biceps or owning a Jaguar. It's a non-verbal statement that says, "I am just the most virile male you will ever meet in your life. I have so much testosterone that I can just throw it away. You definitely want me to be the father of your children."
Recently Paul Zak, "the world's expert on oxytocin," has published further studies (read about them here) that support the claim that oxytocin has a role in more than just sex, pair bonding, and the mother/infant relationship; it's apparently vital in all sorts of positive social interactions. Zak, in fact, calls oxytocin "the moral molecule." His studies indicate that people's oxytocin levels rise when they have pleasant encounters of all sorts; and if given boosts of oxytocin artificially, they tend to make more moral decisions and behave with more generosity and trust. Oxytocin levels also spike, Zak found, when people play with their pets, socialize with their friends, and watch romantic movies with happy endings. All of these are activities that are connected with pair bonding, social cohesion, and reciprocity -- phenomena that are intrinsic to life as a social primate, so no wonder this response is ubiquitous. It'd be a pretty unpleasant world without it, wouldn't it?
Ah, natural selection. It explains so much.
Anyway, I find all of this stuff pretty fascinating, and I wish you luck conducting any empirical research on the subject that you have the opportunity to do. Here's to raised oxytocin levels.