Connection.
If it's a romantic/sexual relationship, to gauge your attraction for the individual and communicate your readiness to step it up a level.
In an affectionate/family relationship, it indicates the levels of that affection, too.
I've read about it having other meanings in other cultures--some Europeans kiss business associates? Or something like that. . . as for why it exists... my perspective on that would be different, as I believe in creation and the immortal soul (not evolution and every human behavior has an animal behavior origin/equivalent). I see humans as uniquely created in the image of God, and believe that God imparted immortal soul into the first human by breathing the pneuma (spirit, wind, breath) physically into his lungs.
So maybe that's why humans kiss. We were made in the image of God, God "kissed" the first man to life, and since then humans kiss those they love and desire a soul-connection with.
You may now proceed to bash the looney right-wing religious conservative for her freaky Biblical views.
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Your neurodiverse (Aspie) score: 141 of 200
Your neurotypical (non-autistic) score: 71 of 200
You are very likely neurodiverse (Aspie)
Official diagnosis: Austism Spectrum Disorder Level One, without learning disability, without speech/language delay; Requiring Support