Bonus link: Thomistic hilarity
“[M]an’s mind would break if its tension were never relaxed. Now such like words or deeds wherein nothing further is sought than the soul's delight, are called playful or humorous. Hence it is necessary at times to make use of them, in order to give rest, as it were, to the soul ....
“Now a man who is without mirth, not only is lacking in playful speech, but is also burdensome to others, since he is deaf to the moderate mirth of others. Consequently [such persons] are vicious, and are said to be boorish or rude.”
-- Thomas Aquinas, Summa Theologiae, II-II, Q. 168, excerpting from Arts. 2 and 3.