Laughter is the best medicine. It is an audible expression or appearance of happiness, or an inward feeling of joy; a part of human behaviour regulated by the brain. It is used as a signal for being part of a group — it signals acceptance and positive interactions with others. Laughter is sometimes seemingly contagious, and the laughter of one person can itself provoke laughter from others as a positive feedback.

"The human race has one really effective weapon, and that is laughter."
~ Mark Twain ~

A general theory that explains laughter is called the relief theory. Sigmund Freud summarized it in his theory that laughter releases tension and "psychic energy". This theory is one of the justifications of the beliefs that laughter is beneficial for one's health; why laughter can be a coping mechanism when you're upset, angry or sad.

"Gelotology" is the study of humor and laughter, its effects on the human body, and of medical abnormalities of laughing. It is also the psychological and physiological study of laughter.  There are many therapies developed around this well-documented and ongoing field of study.

"The most wasted of all days is one without laughter."
~ e e cummings ~