Lip-on-lip kissing is not nearly as universal as we might think it is, so can the diverse number of ways that humans kiss reveal what it is about this intimate act that we find important?
Less than half of all societies kiss with their lips, according to a study of 168 cultures from around the world. William Jankowiak, a professor of anthropology at the University of Nevada Las Vegas, found that only 46% use lip-to-lip kissing in a romantic sense, excluding things like parent-child kissing or greetings.
Two theories for why humans have a need to kiss stem from the idea that as babies we have an innate liking for lip touching. In one case, it might be that we associate lip touching with breastfeeding, and that reflex is innate in everyone.
There is also a suggestion that mothers and their children bond over lip-on-lip kissing because of something called "premastication food transfer". The mothers of our ancient ancestors might have pre-chewed our food for us in our early years, and transferred it directly into our mouths. This is something that is seen in our closest ancestors – chimpanzees – as well as other great apes.
https://www.bbc.com/future/article/20210813-the-reasons-humans-started-kissing