In an article entitled "Why Do We Laugh?" William McDougall discusses
scientifically the value of laughter:

Laughter of man presents a problem with which philosophers
have wrestled in all ages with little success. Man is the only
animal that laughs. And, if laughter may properly be called an
instinctive reaction, the instinct of laughter is the only one
peculiar to the human species....

We are saved from this multitude of small sympathetic pains
and depressions by laughter, which, as we have seen, breaks
up our train of mental activity and prevents our dwelling upon
the distressing situation, and which also provides an antidote
to the depressing influence in the form of physiological
stimulation that raises the blood-pressure and promotes
the circulation of the blood. This, then, is the biological
function of laughter, one of the most delicate and beautiful
of all nature's adjustments. In order that man should reap the
full benefits of life in the social group, it was necessary
that his primitive sympathetic tendencies should be strong and
delicately adjusted. For without this, there could be little
mutual understanding, and only imperfect cooperation and
mutual aid in the more serious difficulties and embarrassments
of life. But, in endowing man with delicately responsive
sympathetic tendencies, nature rendered him liable to suffer
a thousand pains and depressions upon a thousand occasions of
mishap to his fellows, occasions so trivial as to call for no
effort of support or assistance. Here was a dilemma--whether
to leave man so little sympathetic that he would be incapable