4 But wherefore does man bless the Lord at all times? Because he is humble. What is it to be humble? To take not praise unto himself. Who would himself be praised, is proud: who is not proud, is humble. Would you not then be proud? That you may be humble, say what is here written; “In the Lord shall my soul be praised: the humble shall hear thereof and be glad”. Those then who will not be praised in the Lord, are not humble, but fierce, rough, lifted up, proud. Gentle cattle would the Lord have; be thou the Lord's jumentum; that is, be thou humble.
He sits upon you, He rules you: fear not lest you stumble, and fall headlong: that indeed is your infirmity; but consider Who sits upon you. You are an ass's colt, but you carry Christ. For even He on an ass's colt came into the city; and that beast was gentle....“Be not ye as the horse or as the mule, which have no understanding.” For horse and mule sometimes lift up their neck, and by their own fierceness throw off their rider. They are tamed with the bit, with bridle, with stripes, until they learn to submit, and to carry their master. But you, before your jaws are bruised with the bridle, be humble, and carry your Lord: wish not praise for yourself, but praised be He who sits upon you, and say thou, “In the Lord shall my soul be praised; the humble shall hear thereof, and be glad.”...
Source: The Enarrations, or Expositions, on the Psalms (New Advent)