I don’t know, but I remember my grandmother saying that it is good because it makes your lungs bigger! OK, this is not science, but it’s still fun. Don’t be afraid of laughing too much, I don’t think there is a real danger. Although I heard of a guy that once said a joke and started laughing so much that he couldn’t stop for 3 days and then he died…but probably that’s only a myth 😉
That’s a very good question, and what’s interesting is that no other animals laugh (as far as we know) – some (like hyenas) make noises that sound like laughing, but it’s not real laughter, from finding something funny. So why humans laugh is a really interesting question, but I’m afraid I don’t know the answer.