“Why, O Lord,” says he, “have You withdrawn afar off?”. Then he who thus inquired, as if all on a sudden he understood, or as if he asked, though he knew, that he might teach, adds, “You despise in due seasons, in tribulations:” that is, You despise seasonably, and causest tribulations to inflame men's minds with longing for Your coming. For that fountain of life is sweeter to them that have much thirst. Therefore he hints the reason of the delay, saying, “Whilst the ungodly vaunts himself, the poor man is inflamed”. Wondrous it is and true with what earnestness of good hope the little ones are inflamed unto an upright living by comparison with sinners. In which mystery it comes to pass, that even heresies are permitted to exist; not that heretics themselves wish this, but because Divine Providence works this result from their sins, which both makes and ordains the light; but orders only the darkness, that by comparison therewith the light may be more pleasant, as by comparison with heretics the discovery of truth is more sweet. For so, by this comparison, the approved, who are known to God, are made manifest among men.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)