2 Now, why, my beloved, did these pastors first feed the sheep, and were then chosen to be pastors of men? Clearly that they might learn how a pastor cares for his sheep, and is watchful and toils in behalf of his sheep. And when they had learned the manners of pastors, they were chosen for the pastoral office. Jacob fed the sheep of Laban and toiled and was vigilant and led them well; and then he tended and guided well his sons, and taught them the pattern of pastoral work.
And Joseph used to tend the sheep along with his brethren; and in Egypt he became guide to a numerous people, and led them back, as a good pastor does his flock. Moses fed the sheep of Jethro his father-in-law, and he was chosen from (tending) the sheep to tend his people, and as a good pastor he guided them. Moses bore his staff upon his shoulder, and went in front of his people that he was leading, and tended them for forty years; and he was vigilant and toiled on behalf of his sheep, a diligent and good pastor.
When his Lord wished to destroy them because of their sins, in that they worshipped the calf, Moses prayed and besought of his Lord and said:— Either pardon the people for their sins, or else blot me out from Your book that You have written. That is a most diligent pastor, who delivered over himself on behalf of his sheep. That is an excellent leader, who gave himself in behalf of his sheep. And that is a merciful father who cherished his children and reared them up.
Moses the great and wise shepherd, who knew how to lead back the flock, taught Joshua the son of Nun, a man full of the spirit, who (afterwards) led the flock, even all the host of Israel. He destroyed kings and subdued the land, and gave them the land as a place of pasturage, and divided the resting-places and the sheepfolds to his sheep. Furthermore, David fed his father's sheep, and was taken from the sheep to tend his people. So he tended them in the integrity of his heart and by the skill of his hands he guided them. And when David numbered the flock of his sheep, wrath came upon them, and they began to be destroyed.
Then David delivered himself over on behalf of his sheep, when he prayed, saying:— O Lord God, I have sinned in that I have numbered Israel. Let Your hand be on me and on my father's house. These innocent sheep, in what have they sinned? So also all the diligent pastors used thus to give themselves on behalf of their sheep.
Source: Demonstrations (New Advent)