Looking at it objectively, sharing saliva with someone else is a pretty gross thing to do. In fact, we transfer approximately 80 million bacteria for every ten seconds we're kissing each other.
We share 80 million bacteria when we kiss each other — here's why we enjoy it anyway
Kissing is one of our favourite disgusting activities.
The majority of these germs are totally benign, so it's nothing to worry about.
But it's still weird to think about inviting someone to share their spit with you, so why do we do it?
First of all, it makes us feel good. Our lips are packed full of receptor cells, which make them very sensitive. In fact, along with fingertips, they are thought to have the highest concentrations of receptor cells.
When you enjoy kissing someone, these receptors shoot signals to your brain, and you release chemicals like dopamine, which fuels your reward system, and makes you want to carry on kissing.
Endorphins, your body's natural painkillers, are also released, which enhance the feeling of pleasure. If it's a really good kiss, your brain may also release oxytocin, the "love hormone," which makes us feel warm and cuddly, and increases our attachment to the other person.
Male saliva contains measurable amounts of testosterone, which could also increase your libido (if you're kissing a man.)
Dr Sarah Johns, an expert in human reproduction and evolutionary psychology at the University of Kent, told The Independent that as well as being an emotion-driven act, kissing helps us pick our most compatible partner.
"Humans don't have strong olfactory skills and kissing allows you to smell and taste a person and see if you have different immune responses as we tend to feel more attracted to someone with a different immune response," she said.
"The major histocompatibility complex is detectable in body odour, so by kissing and tasting someone it gives the opportunity to assess how similar or different that individual is to you biochemically."
In other words, somehow your body may be able to detect whether reproducing with the person you're kissing would be an evolutionary risk or not.
She added that feeling arousal can inhibit feelings of disgust, meaning we don't necessarily think of all the gross things we're doing while we're doing them, because we're too turned on.
Nobody really knows where kissing came from
There's some debate about whether we started kissing each other for cultural reasons, or if it's something we evolved to do biologically.
About 90% of human populations kiss in some way or another, with the majority of others doing similar things in replacement such as rubbing noses, suggesting it could be something instinctual.
Kissing also isn't unique to humans. Primates such as bonobos often kiss each other, and cats and dogs lick and groom one another.
Some scientists believe that kissing could have evolved from "kiss-feeding" behaviours, which is when mothers pass food from their own mouths to their offspring. Birds still do this with their chicks. One theory is that over time, pressing lips became known as an act of caretaking and love.
According to evolutionary psychologists at the University of Albany, the way men and women feel about kissing can differ quite significantly.
In a study of 1,041 college students, the researchers found that women placed more emphasis on kissing, seeing it more as a deal-breaker. They were more likely than men to insist on kissing before having sex, and emphasising the importance of kissing during and after sexual encounters.
Men were more happy to have sex without kissing, and weren't that bothered about whether their partner was a good kisser or not. They were also more likely than women to initiate french kissing (with tongues.)
So whether it's an evolutionary thing, or just something we've picked up, humans really enjoy kissing. It might be gross on paper, but it looks like the benefits outweigh the costs on this one.
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