A kiss is a sign of affection that 90% of people on this planet have enjoyed. A kiss can signal "welcome", "goodbye" and many variations of "I'm really glad to be close to you". A kiss shared between two people generally signals a desire for more of each other's smell, taste and feel. Research has shown that women are stimulated faster by kissing than any other physical activity before engaging in sex.
It is interesting to recognize that humans are the only species with lips that are turned outside the mouth. What we see as human lips are really part of the inside mouth. No other animals have this much lip exposed. This characteristic in humans, especially in female humans, apparently has evolved through preferential selection for this feature.
Full lips are one of the female features emphasized with makeup and even injections of silicone or one's own fat from other places on the body. Women know that men unconsciously recognize full lips as a sexual signal. The more sexual hormones a woman has, the more full her lips are.
Lips in both men and women swell with additional blood when they become aroused. (As well as other body parts!) As men and women age, their lips become thinner because their sexual hormones decrease in later years.
Various mouth gestures also show affection and desire, such as nuzzling, licking, playful biting and so on. These actions can pleasure the senses by the release of endorphins in the brain that bring two people emotionally closer. Touch, taste and smell sensory nerves are all working to feed pleasure signals to the brain from numerous parts of the body. The lips and tongue have an exceptionally large number of nerves compared to most other areas on the body. The whole body becomes more sensitive. As pheromones are picked up from the partner's skin and hair, strong signals shoot to the old brain limbic system where hormones are released preparing the body for mating and at least temporary bonding.
Kissing that includes oral play is recognized almost worldwide as sexually meaningful signals. A kiss on the mouth, with tongue action, is often used to communicate the emotional connection that is prerequisite to an intimate mating relationship. Some mammals "kiss" before mating as a way of stimulating a partner's maternal instincts. Dolphins nibble, cats give playful bites, dogs lick faces or nuzzle flanks and chimps press lips in their courtship.
During courtship there is a time when the relationship should be intensified and that is usually done with a kiss. It has been said: "Kissing is upper persuasion for lower invasion." If the kiss is both given and received correctly and enjoyably then more prolonged kissing and touching are a natural follow up. This oral action stimulates numerous hormones in both partners. The hormones are not only released from each partner's own brain but also hormones and pheromones are exchanged through the lips, mouth and tongue.
This exchange of body chemicals stimulates each other and causes strong physiological changes in both partners. Additional blood flows to selected parts of the body. Even the ear lobes enlarge with extra blood. Humans are the only species that have ear lobes. It appears that ear lobes have evolved only on humans and that they are body arousal points. (Now that the secret is out, some "crank" is going to demand everyone hide their ear lobes with ear brassieres!)
Physiological changes take place all over the body from kissing. Sensitivity and sensations of pleasure feelings increase throughout the body. The feeling of pain is suppressed. Wonderful tensions increase and there is an overpowering craving for release and satisfaction. This wonderful, (add your own descriptive words here) pleasure and relief from the mating act is the powerful drive that has kept the species from dying off.
Unfortunately, kissing does not work well by yourself so it is best to find a partner. Finding a willing healthy partner is the problem. And where do you go after the kissing gets boring?
If he or she whom you are kissing is normally healthy, 95 percent of the bacteria swapped are harmless and naturally occurring in both of your bodies. Probably all the "foreign" and unfriendly bacteria picked up from your partner would be identified by your own immune system and soon exterminated. It is likely that you may even benefit from "swapping spit" by picking up some new friendly bacteria that can help your body's operation by assisting in digestion and metabolism of food, synthesizing vitamins and helping to eliminate waste materials. Hopefully the health benefits from kissing will outweigh the possibility of catching something negative from your partner. (That is what you are hoping for, right?)