The last thing that makes people laugh namely people want to be friendly because smiles and laughter are the first thing that impressed. So people laughed to build a good friendship and want to make others feel good such as I smiles to friends and say hi with new friend for make a good friendship and then we can talk everything because we are friend.
When I met parents of friends or party with friends I talk and always laughing in order to make a good relationship that effect a good feeling for me.