One problem many Christians face is being too serious. When battling the powers and principalities of this world, the burden thereof has made Christians into a dour race. This lack of mirth is serious, for it seems to fly in the face of many authoritative positions that prescribe an opposite trajectory.
Take for instance the great medieval Doctor of the Church, Thomas Aquinas, who in his Summa noted that a person unable to say anything funny was morally unsound. Much, much later, GK Chesteron wrote that the test of a good religion depends on whether one could joke about it.
However, we find the most insightful and most acutely theological observation from Pope Benedict XVI, who when as Cardinal Joseph Ratzinger, wrote in his Principles of Catholic Theology
[W]e might formulate this basic rule: where joylessness reigns, where humour dies, the spirit of Jesus Christ is assuredly absent. But the reverse is also true: joy is a sign of grace. One who is cheerful from the heart…cannot be far from the God of the evangelium, whose first word on the threshold of the New Testament is ‘Rejoice’!