Why do humans kiss?

Why do we kiss? Good question. Hmmm... From Scienceline:

A big question is whether kissing is learned or instinctual. Some say it is a learned behavior, dating back to the days of our early human ancestors. Back then, mothers may have chewed food and passed it from their mouths into those of their toothless infants. Even after babies cut their teeth, mothers would continue to press their lips against their toddlers’ cheeks to comfort them.

Supporting the idea that kissing is learned rather than instinctual is the fact that not all humans kiss. Certain tribes around the world just don’t make out, anthropologists say. While 90 percent of humans actually do kiss, 10 percent have no idea what they’re missing.

Others believe kissing is indeed an instinctive behavior, and cite animals’ kissing-like behaviors as proof. While most animals rub noses with each other as a gesture of affection, others like to pucker up just like humans. Bonobos, for example, make up tons of excuses to swap some spit. They do it to make up after fights, to comfort each other, to develop social bonds, and sometimes for no clear reason at all – just like us.