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Fun fact: Humans are the only animals with lips that purse outward. You could take that as proof that we're made to kiss. (Some apes do too, but not the kind of make-out sessions us Homosapiens dig.)

So why do we kiss? Research suggests a little smooching helps your brain gather all sorts of important information about the guy (or gal) with whom you've locked lips. It also perks up your senses and prepares your body for that other thing-the one that sometimes follows passionate kissing.

Read on for all the juicy (but not slobbery) details.

Before Your Lips Touch His

Just anticipating a kiss, whether you're wrapping up a great first date or making eyes at a guy across the room, can fire up your brain's reward pathways, explains Sheril Kirshenbaum, author of The Science of Kissing. "The more anticipation you feel leading up to a kiss, the greater the dopamine spike," she says, referring to the pleasure hormone your brain produces when you experience something enjoyable. Dopamine energizes your brain and senses, and prepares them to fully absorb new experiences and sensory information, Kirshenbaum says.

Anticipating the kiss may also trigger a release of norepinephrine in your noodle, she explains. This stress hormone explains the nervousness you experience as his eyes find yours and he starts to lean in.

During the Kiss

Your lips comprise one of your body's densest zones of nerve endings, allowing you to detect even the faintest whisp of sensation, Kirshenbaum says. And thanks to all those nerve endings, kissing fires up a surprisingly large portion of your brain, she says. (Believe it or not, more of your noodle is activated during kissing than during sex, some research suggests.)

Why? Kirshenbaum says one answer might have to do with all the judging your brain is doing as it weighs whether or not you should take things beyond the kiss and into the bedroom. "We're so aware of everything that's happening during a kiss because it's such an important part of the decision-making process when choosing a mate," she explains. "People describe ‘getting lost' in sex. But that's not the case with kissing because our brains are hyper-focused on whether or not to take things further."

Kirshenbaum says women typically have stronger senses of smell than men. And as you kiss, your nose is sniffing around your partner for important scent-based information. This info is delivered in the form of pheromones, chemicals his body secretes that tell your brain all sorts of important things about him, including stuff about his genetic makeup.

One study from Switzerland found women are more attracted to the scents of men whose immunity-coding genes do not match their own. In terms of reproduction, mixing different immunity genes will make your offspring more resistant to disease, the study authors say. (Interesting and related: Kirshenbaum says more research has shown the opposite is true for women on birth control. If you're on the pill, you're more likely to go for a guy whose genetic profile matches your own. She can't say why this is the case, but she and other researchers suspect this could explain why some long-term couples split once the woman stops taking birth control.)

Since your brain is doing it's very best during your kiss to decide whether your tonsil tennis partner is a good fit for you in reproductive terms, it's not uncommon for women to experience a reversal of interest after locking lips.

After Your Kiss

Dopamine is also associated with addiction and habit-forming behaviors, Kirshenbaum says. This may explain why, in the days and weeks after your first (and subsequent) make-out sessions, you just can't seem to get your new partner out of your head. Dopamine can also wipe out your appetite and make it hard to sleep, research shows.

Studies have also found kissing triggers the release of the neurotransmitter serotonin, which causes feelings of obsession. Another hormone, oxytocin, also spikes during and after your kiss. This fosters feeling of affection and closeness, and so keeps you coming back for more even after the initial high has worn off, Kirshenbaum says.

"Kissing is a universal human behavior for a lot of reasons," she says, adding that it's probably one of the most important aspects of our mate selection process. So pucker up!