14 “For you have proved us, O God; You have fired us as silver is fired”. Have not fired us like hay, but like silver: by applying to us fire, You have not turned us into ashes, but You have washed off uncleanness, “You have fired us, as silver is fired.” And see in what manner God is angry against them, whose Soul He has set unto life. “You have led us into a trap:” not that we might be caught and die, but that we might be tried and delivered from it. “You have laid tribulations upon our back.”
For having been to ill purpose lifted up, proud we were: having been to ill purpose lifted up, we were bowed down, in order that being bowed down, we should be lifted up for good. “You have laid tribulations on our back:” “You have set men over our heads”. All these things the Church has suffered in sundry and various persecutions: She has suffered this in Her individual members, even now does suffer it. For there is not one, that in this life could say that he was exempt from these trials.
Therefore there are set even men over our heads: we endure those whom we would not, we suffer for our betters those whom we know to be worse. But if sins be wanting, a man is justly superior: but by how much there are more sins, by so much he is inferior. And it is a good thing to consider ourselves to be sinners, and thus endure men set over our heads: in order that we also to God may confess that deservedly we suffer. For why do you suffer with indignation that which He does who is just?
“You have laid tribulations upon our back: You have set men over our heads.” God seems to be angry, when He does these things: fear not, for a Father He is, He is never so angry as to destroy. When ill you live, if He spares, He is more angry. In a word, these tribulations are the rods of Him correcting, lest there be a sentence from Him punishing....
Source: The Enarrations, or Expositions, on the Psalms (New Advent)