Matthew 9:13 (ESV) … “Go and learn what this means: ‘I desire mercy, and not sacrifice.’ For I came not to call the righteous, but sinners.”
A doctor goes where sick people are. The reason he’s a doctor is to help the sick.
by Inspiration (9:13a)
“Go ye and learn,” the Lord added, applying a well-known rabbinic formula used by Jewish teachers when confronted with superficial knowledge. His critics thought they knew so much about the law, and Jesus said, “Go ye and learn”! He directed them to Hosea 6:6 and repeated the gist of it: “I will have mercy, and not sacrifice.”
by Implication (9:13b)
The reference to Hosea was appropriate, for that book deals with spiritual adultery and harlotry. The burden of the book grew out of the tragedy in Hosea’s family life, and the prophet’s task was to show Israel the heart of God, broken over their hardness and sin. God was saying to Israel, “You bring Me sacrifices as though I had some need for them. What I want is to show you mercy.”
Jesus, talking to those who thought they were teachers, was saying, “Go and learn the heart of God. Then you will understand why I sit with publicans and sinners.” Summarizing His answer to the hypocritical Pharisees, the Lord said, “I am not come to call the righteous, but sinners to repentance” (Matthew 9:13).
 Phillips, J. (2014). Exploring the Gospel of Matthew: An Expository Commentary (Mt 9:12–13b). Kregel Publications; WORDsearch.