Sheril Kirshenbaum, science writer and author of The Science of Kissing, has an interesting discussion on why we kiss and how kisses work to stimulate chemistry between two people:
A kiss puts two people in very close proximity. Our sense of smell allows us to pick up subconscious clues about the other person’s DNA or reproductive status. Biologist Claus Wedekind found that women are most attracted to the scent of men who have a very different genetic code for their immune system in a region of DNA known as the major histocompatibility complex. Pairing off with a male who has a different set of genes for immunity can lead to children that will have a higher level of genetic diversity, making them healthier and more likely to survive. (However, it’s important to note that women who take the birth control pill exhibit the opposite preference.) So even though we may not be consciously aware of it, we use behaviors like kissing to judge whether to take a relationship further, based on genetic evidence. In this manner, it’s fair to say that the act of kissing serves as nature’s litmus test.
You can read the entire piece here. It's a great quick read for your afternoon—which will hopefully include a kiss at some point.