In human affairs whatever is against reason is a sin. Now it is against reason for a man to be burdensome to others, by offering no pleasure to others, and by hindering their enjoyment. Wherefore Seneca [Martin of Braga, Formula Vitae Honestae: cap. De Continentia] says (De Quat. Virt., cap. De Continentia): "Let your conduct be guided by wisdom so that no one will think you rude, or despise you as a cad." 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 they are vicious, and are said to be boorish or rude, as the Philosopher states (Ethic. iv, 8).
[Thomas Aquinas, ST 2-2.168.4]
