Ask a grown-up: why do humans kiss?
There are two theories about why we kiss. The first is that it helps us assess whether a new mate is someone we want to have a long-term relationship with. Kissing lets us get close to the other person. We can then smell and taste their pheromones, which carry information about how healthy they are and whether they are genetically compatible with us. The idea of romantic relationships, from an evolutionary perspective, is that you'll have a baby with that person, so this information is important.
The second reason we kiss is to help us maintain relationships, romantic ones or with family and friends. Our lips are full of nerve endings that are stimulated when you kiss someone, so you get a release of feel-good chemicals in the brain. You think, "Oooh, this is lovely" and it bonds you to the person you're kissing.
Chimps kiss, but not with the same intensity. All humans kiss. In cold climates they might do other things that help them to get close, such as rub noses, but they still kiss, too.
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