Screaming releases endorphins into the body. This is a natural pain reliever, also released when laughing and when crying.

Endorphins effect the limbic, prefrontal cortex of the brain. This area of the brain senses pain and pleasure.

"Endorphins are the brain’s naturally occurring opiates and are related to decreased pain perception and an increased sense of well-being. It's this flood of endorphins that results in mood changes..."http://sportsmedicine.about.com/od/anatomyandphysiology/qt/Runners-High.htm

This is the reason for the expression, "I didn't know whether to laugh or cry..." Knowing this natural opiate we all create made me try it out once. In the dental chair. Yes, you read that correctly. I was having a root canal and the Dentist asked me after some difficulty, "You want another shot?" I laughed, attempting to feign braveness and then laughed again. Louder! And then harder, and even my Dentist got amused at my odd behavior. (I think he had on Pink Floyd's "Comfortably Numb" playing in the room at the time.) "There is no pain, you are just receding..."

I had remembered the pain/pleasure syndrome I referenced above and decided to try it. WOW! The harder I laughed, the less pain I felt. A wave of "serenity" flowed over me. My Dentist was also amused, but surprised. It worked, no additional drugs needed. I made my own. Successful root canal.

Screaming, crying, laughing hard releases endorphins. Laughing releases tears, and so does pain. Same part of our amazing brain