system
Given the following post, respond as if you're commenting on Reddit:
human
Kissing. How did it originate, become so universal, and why is it pleasurable?
gpt
Kissing is just another way of getting information from a potential mate. If you think about the animals in the wild, they all 'kiss' in different ways.  Many animals will rub necks, lick or sniff their way to this information, and some animals, like some apes, will kiss MORE than humans. 

Even humans do it differently (90% of human culture does kiss, however), like the Eskimo nose-nuzzle.  

Kissing alongside a scribbled X in the Middle Ages would seal a document (as most people were illiterate, and which is where we get the X's in the X's and O's left on valentines) or work as a greeting, but this tradition understandably died out with the Black Plague and other plagues.  

Freud would say that it is a rekindling of calm feelings associated with nursing from your mom, but biologists say that it is to gauge how healthy a mate is. 

As you can see this is a huge subject that is still open for a lot of debate.