12 But that those prayers, the blessings of which surpass all words, may be more fervent and more constant, the gift that shall last unto eternity is deferred, while transitory evils are allowed to thicken. And so it follows: “Lord, why have You cast off my prayer?”, which may be compared with another Psalm: “My God, My God, look upon me; why have You forsaken me?” The reason is made matter of question, not as if the wisdom of God were blamed as doing so without a cause; and so here. “Lord, why have You cast off my prayer?” But if this cause be attended to carefully, it will be found indicated above; for it is with the view that the prayers of the Saints are, as it were, repelled by the delay of so great a blessing, and by the adversity they encounter in the troubles of life, that the flame, thus fanned, may burst into a brighter blaze.
Source: The Enarrations, or Expositions, on the Psalms (New Advent)