Seems rather interesting to think about how it became an exchange between lovers. The mouth is a rather intimate place - the lips and tongue are full of nerve endings. The lips in particular tend to become engorged and pinker during arousal (and yeah, they look like genitalia - so no wonder women are always trying to make them look bigger and plumper via injections, or even lipstick to emphasize the color.)
But even if you strip that away, the mouth is so very important to the human animal. You eat and drink with it. You breathe with it. Taste with it. Clean mouths and gums are signs of health and vitality - all very important at the subprimal level when choosing a mate. To place your mouth on another person's is a intimate form of trust and comfort, never mind that that science says our saliva swaps tell us things on a molecular level that we probably don't really comprehend. Pheromones, you know.
Still, kissing can be much more than that. A buss upon the cheeks is often a sign of social greeting and shared status, or perhaps a sign of respect (i.e. kissing the ring of the king)...or even symbolize a betrayal (i.e. the kiss of Judas)
The ancient people of India believe that the soul could escape from your breath - when you kissed another person upon the mouth you were giving your souls a chance to mingle. (I like this idea best, from a romantic standpoint. Far more interesting than pheromones.)
For fun, I'll leave you with the first kiss excerpt between Abby and Brystion from A Brush of Darkness. Seems fitting with the paragraph up above since he's an incubus...and consuming souls is sort of what they're known for...
“I wasn’t trying to seduce you. The offer was genuine.” He pushed his hand through his hair. “It’s a little disconcerting to be turned down by a mere mortal.”
“Just a mortal,” I snorted. “Real nice. I’ll see what I can do about soothing your ego, O gracious and tactful one.” I chewed on my lower lip thoughtfully and gave him a sly smile. “Of course, you probably shouldn’t feel too bad. After all, I am wearing a magical amulet now.”
His mouth pursed. “You are?”
“Sure. It’s made of silver and moonbeams and blessed by a flatulent dwarf,” I intoned gravely. “It’s a guaranteed ‘plus four’against Incubus Seduction.”
“You’re an ass.”
“Kiss, kiss, darling.” I fluttered my eyelashes, puckering my lips in mock affection.
His hand snarled into my hair, fingers twined tightly at the base of my head. “This is a complication I don’t want, Abby.” He growled the words, but there was no mistaking the desire that smoldered behind his now glowing eyes.
“The bulge in your pants says otherwise,” I retorted, perversely nudging my hips against him. A little voice in the back of my mind was going into apoplectic fits at my boldness. As far as I was concerned, the incubus had been acting like some sort of preternatural cocktease since we’d met and I’d had enough.
He let out a stifled groan, his other hand snaking down to grip my ass. “You’ll regret it,” he breathed, releasing his hold on my hair to trace a curious thumb over my jaw. His face drifted closer until his mouth brushed mine. I shuddered at the delicate intrusion. His fingers slid up to the small of my back.
“Probably.” I sighed, my mind happily unable to focus on anything but the way he was nipping at my lower lip. “I regret a lot of things.”
The incubus stared at me, an unnamed emotion flickering across his face, and then his lips were on mine, fierce and possessive. He devoured me utterly. There was only the sweetness of his tongue, probing hot and wet into the velvet contours of my mouth. It swept shallow, lingering to taste the soft edges, and then moved deeper, pulsing and rhythmic to match the rapid beating of my heart. I jerked forward to bury my hands in his hair, my ragged breathing giving way to a low cry of longing.
“How’s that ego?” he purred.
“Rock hard from the feel of it,” I gasped. “Just the way I like it.”
