The image of the Lord as shepherd occurs in the Old Testament as well as the New. Psalm 23 describes the care of the shepherd for his sheep. He guides them, nourishes them, soothes them and protects them. This a a good description of Jesus, who described himself as the Good Shepherd.
Palestine in Jesus' time was a colony of Rome. Pontius Pilate had a reputation as a cruel governor who frequently acted sacrilegiously against the Jews, including mixing the blood of the Galileans with their sacrifices (Luke 13:1-5). Herod Antipas was a Roman puppet, unpopular with his people and scandalous in his private life. The priestly families, the Pharisees and Saducees were mostly more concerned with their own status, and often were mercenary as well. Jesus, therefore, was providing a totally different leadership, which was unselfish and caring. He saw the people with these unscrupulous rulers as sheep without a shepherd (Matt. 9:36)
As sheep follow a shepherd, so Christians must follow the example of their Lord, so that his values become part of the community which is his body. In Acts, Peter - like Christ - was fearless in speaking the truth; and we are fortunate that Pope Leo is doing the same.