Kissing has historically been used as a symbol of sealing contracts, and can be seen in the medieval practices of sealing feudal relationships and in sealing marriages with a kiss. People who could not write their own names would put an "X" at the bottom of written documents like contracts (representing the kiss as it does in the XOXO of today) to symbolize their agreement to the contents of the document.

hi! this question may be worth cross-posting to /r/AskScience or /r/AskAnthropology, but meanwhile, you may be interested in these previous discussions from this sub

• Was oral hygiene, or lack thereof, ever a deterrent for people to kiss before contemporary methods became available?

• What are the origins of kissing as a romantic act?

• When and where did the act of kissing started having sexual undertones?

• When did people start kissing each other on the mouth as a show of romantic affection? What is the history of "French" (with tongue) kissing?