And it's true. The saying was not just created out of thin air.
With my local Headway, I am attending a cognitive thinking course at the moment and I am learning quite a bit about the brain and experiences from others.
The lady running the group, and our current manager, said that she read about a guy who had/has a brain injury and he cured it/aided his injury, don't know all the facts, by making himself laugh a lot. Watching funny things and so on.
When you laugh, the brain creates a certain chemical. So obviously, the more you laugh, the more chemical which helped with his recovery.
Again, nature shows how powerful it can be :).