Objection 1. It would seem that there is no sin in lack of mirth. For no sin is prescribed to a penitent. But Augustine speaking of a penitent says (De Vera et Falsa Poenit. 15) [Spurious]: "Let him refrain from games and the sights of the world, if he wishes to obtain the grace of a full pardon." Therefore there is no sin in lack of mirth.
Objection 2. Further, no sin is included in the praise given to holy men. But some persons are praised for having refrained from mirth; for it is written (Jeremiah 15:17): "I sat not in the assembly of jesters," and (Tobit 3:17): "Never have I joined myself with them that play; neither have I made myself partaker with them that walk in lightness." Therefore there can be no sin in the lack of mirth.
Objection 3. Further, Andronicus counts austerity to be one of the virtues, and he describes it as a habit whereby a man neither gives nor receives the pleasures of conversation. Now this pertains to the lack of mirth. Therefore the lack of mirth is virtuous rather than sinful.
On the contrary, The Philosopher (Ethic. ii, 7; iv, 8) reckons the lack of mirth to be a vice.
I answer that, 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).
Since, however, mirth is useful for the sake of the rest and pleasures it affords; and since, in human life, pleasure and rest are not in quest for their own sake, but for the sake of operation, as stated in Ethic. x, 6, it follows that "lack of mirth is less sinful than excess thereof." Hence the Philosopher says (Ethic. ix, 10): "We should make few friends for the sake of pleasure, since but little sweetness suffices to season life, just as little salt suffices for our meat."
Reply to Objection 1. Mirth is forbidden the penitent because he is called upon to mourn for his sins. Nor does this imply a vice in default, because this very diminishment of mirth in them is in accordance with reason.
Reply to Objection 2. Jeremias speaks there in accordance with the times, the state of which required that man should mourn; wherefore he adds: "I sat alone, because Thou hast filled me with threats." The words of Tobit 3 refer to excessive mirth; and this is evident from his adding: "Neither have I made myself partaker with them that walk in lightness."
Reply to Objection 3. Austerity, as a virtue, does not exclude all pleasures, but only such as are excessive and inordinate; wherefore it would seem to pertain to affability, which the Philosopher (Ethic. iv, 6) calls "friendliness," or eutrapelia, otherwise wittiness. Nevertheless he names and defines it thus in respect of its agreement with temperance, to which it belongs to restrain pleasure.
Source: Summa Theologica (New Advent)