"Ultimately, sarcastic put-down humor is self-righteous —a form of self-justification— and that is what the
gospel demolishes. When we grasp that we are unworthy sinners saved by an infinitely costly grace, it destroys both our self-righteousness and our need to ridicule others. This is also true of self-directed ridicule.
Some people constantly and bitterly mock themselves. At first it looks like a form of humility, or realism, but
really it is just as self-absorbed as the other version. It is a sign of an inner discomfort with one’s self, a
profound spiritual restlessness.
There is another kind of self-righteousness, however, that produces a person with little or no sense of humor. Moralistic persons often have no sense of irony, because they take themselves too seriously or because they are too self-conscious and self-absorbed in their own struggles to be habitually joyful. The gospel, however, creates a gentle sense of irony. Our doctrine of sin keeps us from being over-awed by anyone (especially ourselves) or shocked by any behavior. We find a lot to laugh at, starting with our own weaknesses. They don’t threaten us anymore, because our ultimate worth is not based on our record or performance. Our doctrine of grace and redemption also keeps us from seeing any situation as hopeless. This “ground note” of joy and peace makes humor spontaneous and natural."
Tim Keller, The Gospel and Humour