laughings really fun isnt it? isnt it great feeling when there's something so funny that you can't stop and you can't stand up straight because your belly hurts and you're crying? thats a fantastic feeling!
Apparently it's all about social bonding and making and strengthening human connections. It's been researched quite extensively. Check out this link for all the blurb:
small children laugh or giggle when they get tossed up in the air by an adult, resulting from turbulence in the child's developing brain. It's just a physical expression.