16 “Be glad, you righteous”. Perhaps already the faithful hearing the word, “Be glad,” are thinking of banquets, preparing cups, waiting for the season of roses; because it is said, “Be glad, you righteous!” See what follows, “Be glad in the Lord.” You are waiting for the season of spring, that you may be glad: you have the Lord for joyful gladness, the Lord is always with you, He has no special season; you have Him by night, you have Him by day. Be true-hearted; and you have ever joy from Him.
For that joy which is after the fashion of the world, is not true joy. Hear the prophet Isaiah: “There is no joy, says my God, to the wicked.” What the wicked call joy is not joy, such as he knew who made no account of their joy: let us believe him, brethren. He was a man, but he knew both kinds of joy. He certainly knew the joys of the cup, for he was a man, he knew the joy of the table, he knew the joys of marriage, he knew those joys worldly and luxurious. He who knew them says with confidence, “There is no joy to the wicked, says the Lord.”
But it is not man who speaks, it is the Lord....But you say, I see not that light which Isaiah saw. Believe, and you shall see it. For perhaps you have not the eye to see it; for it is an eye by which that beauty is discerned. For as there is an eye of the flesh, by means of which this light is seen: so there is an eye of the heart, by which that joy is perceived: perhaps that eye is wounded, dimmed, disturbed by passion, by avarice, by indulgence, by senseless lust; your eye is disturbed: you can not see that light. Believe, before you see; you shall be healed, and shall see.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)