I have read before that grooming a mate evolved among the apes as a means of showing affection. All of the great apes existing today (with the exception of humans) include eating insects off of each other as part of grooming ritual. I wonder if kissing is the homo-sapien substitute for eating bugs off of a mate. It could be the case that the early ancestors of modern humans felt comfort from having their mates nibble potential parasites of their bodies. Perhaps the act of making gentle mouth actions on another's body gradually became associated with care and love. Perhaps this association, no longer needing the ultilitarian purpose of removing parasites, evolved into modern mouth-to-mouth kissing.