Hebrews 5:4-5

4Nobody decides for himself to become a leader of the priests. God himself must choose him. That is how Aaron became a priest.

5It is the same with Christ. He did not choose himself to be God's special priest. He did not choose to make himself important like that. God chose him. God showed that Christ was very important. God said to him: ‘You are my Son. Today I have become your Father.’