Rabbi Moshe Chaim Luzzato, in his book The Path Of The Just, warns us about the effect that levity can have on our spiritual growth:
Consider the great severity and destructive power of levity. Like a shield smeared with oil, which wards off arrows and causes them to fall to the ground, not permitting them to reach the bearer's body, is levity in the face of reproof and rebuke. For with one bit of levity and with a little laughter a person can cast from himself the great majority of the awakenings and impressions that a man's heart stimulates and effects within itself upon his seeing or hearing things which arouse him to an acconting and an examination of his deeds. The force of levity flings everything to the ground so that no impression whatsoever is made upon Him. This is due not to the weakness of the forces playing upon him, nor to any lack of understanding on his part, but to the power of levity, which obliterates all facets of moral evaluation and fear of God.As we search our souls this Elul, levity is one of the things that must be done away with.