Laughter
Overview
Laughing is a reaction to certain stimuli, fundamentally stress
Stress (biology)
Stress is a term in psychology and biology, borrowed from physics and engineering and first used in the biological context in the 1930s, which has in more recent decades become commonly used in popular parlance...

, which serves as an emotional balancing mechanism. Traditionally, it is considered a visual expression of happiness
Happiness
Happiness is a mental state of well-being characterized by positive emotions ranging from contentment to intense joy. A variety of biological, psychological, religious, and philosophical approaches have striven to define happiness and identify its sources....

, or an inward feeling of joy. It may ensue from hearing a joke
Joke
A joke is a phrase or a paragraph with a humorous twist. It can be in many different forms, such as a question or short story. To achieve this end, jokes may employ irony, sarcasm, word play and other devices...

, being tickled
Tickling
Tickling is the act of touching a part of the body so as to cause involuntary twitching movements and/or laughter. The word evolved from the Middle English tikelen, perhaps frequentative of ticken, to touch lightly. The idiom tickled pink means to be pleased or delighted.In 1897, psychologists G...

, or other stimuli. It is in most cases a very pleasant sensation.

Laughter is found among various animals
Laughter in animals
Laughter in animals other than humans describes animal behavior which resembles human laughter.According to Dr. Brian Carroll, self awareness is conscious concomitant of the physiological processes involving laughter or smiling reflex and its grades, degrees or spectrum varies according to...

, as well as in humans, although it is more rare in most mammals and animals overall.
Quotations

You grow up the first day you have your first good laugh — at yourself.

Ethel Barrymore|Ethel Barrymore, as quoted in 1,600 Quotes & Pieces of Wisdom That Just Might Help You Out When You're Stuck in a Moment (and Can't Get Out of It!) (2003) by Gary P. Guthrie

Mirth is God's medicine. Everybody ought to bathe in it.

Henry Ward Beecher, in Royal Truths (1869), p. 248

Whoever undertakes to set himself up as a judge of Truth and Knowledge is shipwrecked by the laughter of the gods.

Edmund Burke in the Preface to Brissot's Address to his Constituents (1794)

And if I laugh at any mortal thing, 'Tis that I may not weep.

Lord Byron, Don Juan (Byron)|Don Juan, Cto. IV, st. 4 (1821)

No man who has once heartily and wholly laughed can be altogether irreclaimably bad.

Thomas Carlyle, Sartor Resartus, Bk. I, ch. 4 (1833–1834)

It is not funny that anything else should fall down, only that a man should fall down.... Why do we laugh? Because it is a gravely religious matter: it is the Fall of Man. Only man can be absurd: for only man can be dignified.

G. K. Chesterton, All Things Considered, "Spiritualism", (1908)

The highest forms of understanding we can achieve are laughter and human compassion.

Richard Feynman, in What Do You Care What Other People Think?|What Do You Care What Other People Think? (1988)

Anyone who takes himself too seriously always runs the risk of looking ridiculous; anyone who can consistently laugh at himself does not.

Vaclav Havel, Disturbing the Peace, Ch. 2 (1986)

Man is the only animal that laughs and weeps; for he is the only animal that is struck with the difference between what things are, and what they ought to be.

William Hazlitt, Lectures on the English Comic Writers, "Lecture I: On Wit and Humour" (1819)

 
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