system
Given the following post, respond as if you're commenting on Reddit:
human
Why do humans kiss?  Are there any other animals that do this to display love, or is there an animal equivalent in some species?

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gpt
It's an open question.  There are hypotheses that it is a learned, cultural behavior, and hypotheses that it is instinctual.  There are certain human tribes that don't kiss, or appear to be aware of the idea; on the other hand, bonobos kiss often in a similar way to humans (and other animals have their own displays of affection, like nose-rubbing or nuzzling).

One "learned behavior" hypothesis postulates that mothers in primitive tribes would chew food for their young infants, bird-style, and then after they develop teeth, continue to "fake" the motions in order to comfort them.  One "instinctual" hypothesis is the idea that kissing enhances the exchange of pheromones between individuals, and so helps individuals decide if another individual is an appropriate mate.  This latter idea is somewhat weakened by the fact that though humans produce pheromones, they don't seem to have much of an effect on us anymore.