Smithsonian: [K]issing helps heterosexuals select a mate. Women in particular value kissing early on. Saliva is full of hormones and other compounds that may provide a way of chemically assessing mate suitability—that’s the biological brain stepping in.
…While kissing, couples exchange 9 milliliters of water, 0.7 milligrams of protein, 0.18 mg of organic compounds, 0.71 mg of fats, and 0.45 mg of sodium chloride, along with 10 million to 1 billion bacteria, according to one accounting.
Women are also more likely to say that a first kiss could be the decider for selecting a mate. Can the biological drive overcome the perception that your chosen one is a bad kisser? Wlodarski says it’s hard to separate the two, but that “I would hazard a guess that if someone thinks someone is a bad kisser it’s because their smell wasn’t right,” he says. Women have to be more selective because they face greater consequences when they make a poor mating decision—like having to carry a baby for nine months, says Wlodarski.
…Not every culture is down with the full-on mouth kissing enlivened by a wandering tongue. That seems to be a modern, and Western, convention, perhaps from the last 2,000 years, says Wlodarski. A study published in 2015 found that less than half of the cultures surveyed engage in romantic, sexual kissing.
There’s evidence—at least from written history—that in the past, kissing was primarily mutual face or nose rubbing, or even sniffing in close proximity. In Hindu Vedic Sanskrit texts, kissing was described as inhaling each other’s soul.