5 And remember, my beloved, that I wrote to you concerning our fathers of old that they first learned the ways of tending sheep and in that received trial of carefulness, and then were chosen for the office of guides, that they might learn and observe how much the pastor cares for his flock, and as they used to guide the sheep carefully, so also might be perfected in this office of guidance. Thus Joseph was chosen from the sheep, to guide the Egyptians in the time of affliction.
And Moses was chosen from the sheep, to guide his people and tend them. And David was taken from following the sheep, to become king over Israel. And the Lord took Amos from following the sheep, and made him a prophet over his people. Elisha likewise was taken from behind the yoke, to become a prophet in Israel. Moses did not return to his sheep, nor did he leave his flock that was committed to him. David did not return to his father's sheep, but guided his people in the integrity of his heart. Amos did not turn back to feed his sheep, or to gather (the fruit of) trees, but he guided them and performed his office of prophecy.
Elisha did not turn back to his yoke, but served Elijah and filled his place. And he who was for him as a shepherd, because he loved fields and merchandise and vineyards and oliveyards and tillage, did not wish to become his disciple; and (therefore) he did not commit the flock into his hand.
Source: Demonstrations (New Advent)