Christianity has a history of being anti-fun. After all, what role could fun have in a sober life of self-denial and service? The medieval scholastic theologian Thomas Aquinas saw that it would be irrational and sinful to exclude fun from the Christian life, because fun is a necessary form of spiritual and mental rest.
Augustine says (Music. ii, 15): “I pray thee, spare thyself at times: for it becomes a wise man sometimes: for it becomes a wise man sometimes to relax the high pressure of his attention to work.” Now this relaxation of the mind from work consists in playful words or deeds. Therefore it becomes a wise and virtuous man to have recourse to such things at times. Moreover the Philosopher [Ethic. ii, 7; iv, 8] assigns to games the virtue of eutrapelia, which we may call “pleasantness.”
I answer that, Just as man needs bodily rest for the body’s refreshment, because he cannot always be at work, since his power is finite and equal to a certain fixed amount of labor, so too is it with his soul, whose power is also finite and equal to a fixed amount of work.Thomas Aquinas, Summa Theologica II-II, q. 168
Aquinas goes on to support the use of fun as spiritual rest from a story told by the desert fathers. The apocryphal story about the Apostle John makes the point that a man can only do his best work if he takes time to rest. Aquinas seems to tell the story inaccurately, so you can read the primary source in the screenshot below:
But play and joking are not only for our own benefit. Though he sees more danger in excessive fun, Aquinas argues that “there is a sin in lack of mirth:”
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 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, Summa Theologica II-II, q. 168
The Bible teaches Christians to live in cycles of work and rest. God ordained night, the Sabbath, and feasts as times to rest. In times of rest we gratefully enjoy God’s creation, and are refreshed so we can gladly go back to work. Jesus understands the Sabbath as a gift to man to bring wholeness:
And he said to them, “The Sabbath was made for man, not man for the Sabbath. So the Son of Man is lord even of the Sabbath.”
Again he entered the synagogue, and a man was there with a withered hand. And they watched Jesus, to see whether he would heal him on the Sabbath, so that they might accuse him. And he said to the man with the withered hand, “Come here.” And he said to them, “Is it lawful on the Sabbath to do good or to do harm, to save life or to kill?” But they were silent. And he looked around at them with anger, grieved at their hardness of heart, and said to the man, “Stretch out your hand.” He stretched it out, and his hand was restored.Mark 2:27-3:5
Just as Christians should rest and let others rest, Christians should have fun so that they and others may be refreshed. The spiritual and mental rest that we enjoy in games and jocular conversation is a gift from God that should be gratefully enjoyed and shared.