Kissing.

In western culture, kissing is seen as a sign of affection. But the reasons for doing it, apart from the fact that it is enjoyable, are not widely known. But as with many apparently inexplicable behaviors, it does indeed have scientific basis.

Kissing is hypothesized to be a form of immune boosting. When saliva is transfered from the typically taller male to the female by gravity, it increases the womans immune response to foreign germs. This is to help to combat sickness during and before pregnancy.

Also is the possibility that by exposing her to his oral bodily fluids, it increases the likely hood of the males spermatozoa being accepted by the female's body after copulation, however this is less probable.

Some cultures, specifically aboriginal African and Australian cultures, did not engage in the activity of kissing until it was introduced by European settler. This shows that it is not an instinctual but, in fact, a learned behavior.

It is perhaps a vestigial behavior in our modern world, but it is fun to engage in and potentially even beneficial.