Laughter is weird - and we do it a lot. One study found that people laugh seven times for every 10 minutes of conversation. It's been found that if you ask people what makes them laugh they'll talk about jokes and humour, but we laugh most frequently when we are with other people - and hardly ever at jokes.
It's a social emotion and we use it to make and maintain social bonds. Laughter is also a very primitive way of making a sound.