Do humans kiss for health benefits? Maybe. An immunology study suggested that kissing is a way of passing on immunity to certain viruses. A nasty bug called cytomegalovirus is dangerous when introduced to pregnant women. The virus can cause birth defects or death, so it is important that women not be exposed to cytomegalovirus while they are pregnant. But what if they are exposed to it before they are pregnant? Then they have a chance to build up immunity to it. Thank you, antibodies.
So if a man finds a woman attractive and might want to impregnate her, it is in his best interest to have her build up some antibodies first to protect their offspring. Enter kissing. Passing germs through saliva is a great way to build up antibodies, and so kissing may have been kept in the selective lineup because it actually has some benefits for health and survival. So next time one of your little cousins says kissing is gross, tell them they are right. Germs are gross.
Keeping the family healthy is also important to other primates, who take turns grooming each other, picking dirt, insects, and leaves out of each other's hair. Practicing good hygiene keeps pathogens away from skin, and it is also a way that chimps create and maintain social ties. Instead of having your friends over to play videogames or watch a movie, next time invite them over for a grooming party. It is sure to be a big hit.