It was always God's plan to exalt the Son, and to give Him all power and authority with a Name above all names.
And 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. That 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."
And in another passage God said to him, "You are a priest forever in the order of Melchizedek."
While Jesus was here on earth, he offered prayers and pleadings, with a loud cry and tears, to the one who could rescue him from death. And God heard his prayers because of his deep reverence for God. Even though Jesus was God's Son, he learned obedience from the things he suffered.
In this way, God qualified him as a perfect High Priest, and he became the source of eternal salvation for all those who obey him. [Hebrews 5:4-9 NLT]
Christ's Sonship—His
eternal Sonship (not to be confused with the Incarnation)—is His calling to Priesthood. His Incarnation in the likeness of
sinful flesh qualified Him.
Being a priest means that one is required to make an offering.
And since every high priest is required to offer gifts and sacrifices, our High Priest must make an offering, too. [Hebrews 8:3 NLT]
But God is not pleased with the blood of bulls and goats.
Wherefore when he cometh into the world, he saith, Sacrifice and offering thou wouldest not, but a body hast thou prepared me: In burnt offerings and sacrifices for sin thou hast had no pleasure.
Then said I, Lo, I come (in the volume of the book it is written of me,) to do thy will, O God.
Above when he said, Sacrifice and offering and burnt offerings and offering for sin thou wouldest not, neither hadst pleasure therein; which are offered by the law; Then said he, Lo, I come to do thy will, O God.
He taketh away the first, that he may establish the second. By the which will we are sanctified through the offering of the body of Jesus Christ once for all. [Hebrews 10:5-10 KJV]