some quick research on laughter-

laughing is built into us. human babies only a few months old laugh. also, the fact that every human can laugh and that laughing is easy to recognize due to its simple structure shows that it is built into us. there is also a condition of pathological laughter in which "damage to a wide variety of brain regions produces abnormal laughter".

by looking at our closest animal relatives we can see that our lineage had laughter from the start. one source states how "chimpanzees and other great apes perform a laugh-like vocalization when tickled or during play." it also states that "chimpanzee laughter occurs almost exclusively during physical contact, or during the threat of such contact, during chasing games, wrestling or tickling."

animals play. you can see their young engaging in chase games or wrestling or whatever else they do. how do you tell play fighting from real fighting? laughing may have come about to further show that play fighting is just play.

in humans though, it may have further developed to make and strengthen human connections. laughing seems to have a social function. "In the absence of stimulating media (television, radio or books), people are about 30 times more likely to laugh when they are in a social situation than when they are alone."

The Evolution of Laughter
How Laughter Works
Laughter