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
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.