Sheril Kirshenbaum of the University of Texas at Austin and the author of "The Science of Kissing," says a kiss is a natural litmus test to help people identify a good partner because it requires two people to get close enough to "sniff out" a reproductive partner, detecting things about a person's health and DNA.
In an article in The Washington Post, Kirshenbaum writes that biologist Claus Wedekind found women are most attracted to the scents of men with a different set of genetic coding for immunity than their own. This is probably because when there is greater genetic diversity between parents, children will have more versatile immune systems and better odds for survival, Kirshenbaum says.
Kirshenbaum explains during a passionate kiss, blood vessels dilate, the brain receives more oxygen, breathing can become irregular and deepen, cheeks flush, pulse quickens and pupils dilate.
Not only can a couple smell each other, they can sample each others' taste, which can also reveal health and fertility clues.
In addition, kissing triggers the "love hormone," oxytocin, which promotes social bonding and maintaining a special connection between two people, long after novelty has waned.
However, a bad kiss triggers the "stress hormone" cortisol and can kill a budding relationship on the spot.