In Cecils laughter is the best medicine answer I was surprised not to hear anything about endorphins. I learned through a psychology course that when you smile endorphins are released in your body and it gives you sort of a ‘natural high’. Of course when you laugh you’re smiling usually. 
endorphins
Pronounced As: endôrfinz , neurotransmitters found in the brain that have pain-relieving properties similar to morphine. There are three major types of endorphins: beta endorpins, found primarily in the pituitary gland; and enkephalins and dynorphin, both distributed throughout the nervous system. Endorphins interact with opiate receptor neurons to reduce the intensity of pain: among individuals afflicted with chronic pain disorders, endorphins are often found in high numbers. Many painkilling drugs, such as morphine and codeine, act like endorphins and actually activate opiate receptors. Besides behaving as a pain regulator, endorphins are also thought to be connected to physiological processes including euphoric feelings, appetite modulation, and the release of sex hormones.