If your're not religious, then forgive my answer. Some of old antiquitous writings have some cool answers. They say the first kiss was God to Adam. God breathed his spirit into Adam making him a living soul. Gnostic writers say that a kiss is the passing of a person's spirit into another when kissed on the lips. Therefore, being so ancient and relevant, is has become second nature to humans to kiss. The deep meanings are hid to us, but is a necessary instinct like a baby's suckling for life. For two people to be one whole, they must share the same spirit and, to some, a kiss is a big part of it.