An important question is whether kissing is acquired or an instinct. Some people say that it is a learning behavior, and our ancestors can kiss. At that time, the mother chewed up the food first and then sent it to the baby's mouth with her mouth. Even if the child has a broken tooth, the mother will still kiss the child's lips to show comfort.

There is a fact that not everyone can kiss, which supports the conclusion that kissing is acquired rather than instinct. Anthropologists say that tribes in many parts of the world don't actually kiss. Statistics show that 90% people kiss, while 10% people don't kiss at all. .

Others believe that kissing is an instinct and take kissing-like behavior of animals as evidence. Most animals show their love for each other by rubbing their noses repeatedly, while other animals like to express their feelings with their mouths just like people. For example, bonobos exchange saliva to express their deep apologies. After the war, they all did this when comforting each other and developing social relations, and sometimes they kissed inexplicably like human beings.

Nowadays, the most recognized reason for human kissing is that it can help us find the right partner. When men and women's faces are close together, pheromones will convey biological information about whether they will have strong offspring. For example, women prefer men whose immune system protein genes are different from their own, which may give birth to offspring with stronger immune system and higher survival probability.