5 But why this? Because He “scourges every son whom He receives.” Why this? Because to men sinning was it said, “In the sweat of your face shall you eat bread.” Therefore because all these chastisements, in which all our bed is turned in our infirmity, man ought to acknowledge that he suffers for sin; let him turn himself, and say what follows: “I said, Lord, be merciful unto me; heal my soul, for I have sinned against You”. O Lord, by tribulations do Thou exercise me; to be scourged You judge every son whom You will receive, who spared not even the Only-Begotten. He indeed without sin was scourged; but I say, “I have sinned against You.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)