Hebrews 5:4-6

4And no one can become a high priest simply because he wants such an honor. He must be called by God for this work, just as Aaron was.

5That is why Christ did not honor himself by assuming he could become High Priest. No, he was chosen by God, who said to him, “You are my Son. Today I have become your Father.”

6And in another passage God said to him, “You are a priest forever in the order of Melchizedek.”