I was a lucky kid. I got very interested in the opposite sex in 5th grade, which gave me early access to one of the favorite activities of my entire life - kissing. One definition of a kiss is to "touch with the lips as a sign of love, sexual desire, reference, or greeting". Well, that about covers it.
There is debate among anthropologists on the evolution of the kiss. One school of thought is that it developed from "kiss feeding" babies; that is, pre-chewed food being passed from mother to child. Others believe that it is instinctual: it's just what we do as human beings and has a multitude of functions. And it is true that our closest relative, the chimpanzee, kisses as well.
And after holding hands, kissing is the next thing we do as adolescents in exploring the mysteries of those we are attracted to. Now not all cultures engaged in kissing but most of them do.
By getting so close to another person you gain a lot of information; their scent, expressions on their face, how healthy they are and their intentions. Tongue kissing -- aka French kissing (which, by the way, the French don't call it that: they say Florentine kiss) -- is of course the more passionate and telling kiss. Our tongues have 10,000 neurons on them. The mouth is incredibly important to survival, not only determining what kind of food to eat but what kind of mates to have. It really is about reproduction. There is also a gigantic exchange of bacteri; 80 million germs in 10 seconds of French kissing, which is thought to enhance immunity to disease.
Kissing also releases oxytocin, which is a calming natural sedative. On the other hand, passionate kissing is, of course, sexually arousing and dopamine, a natural stimulant, is generated. Calories are burned. Self-esteem is boosted. Kissing is something really good for you.