For every high priest chosen from among men is appointed to act on behalf of men in relation to God, to offer gifts and sacrifices for sins. He can deal gently with the ignorant and wayward, since he himself is beset with weakness. Because of this he is obligated to offer sacrifice for his own sins just as he does for those of the people. And no one takes this honor for himself, but only when called by God, just as Aaron was.
So also Christ did not exalt himself to be made a high priest, but was appointed by him who said to him,
“You are my Son,
today I have begotten you”;
as he says also in another place,
“You are a priest forever,
after the order of Melchizedek.”
In the days of his flesh, Jesus offered up prayers and supplications, with loud cries and tears, to him who was able to save him from death, and he was heard because of his reverence. Although he was a son, he learned obedience through what he suffered. And being made perfect, he became the source of eternal salvation to all who obey him, being designated by God a high priest after the order of Melchizedek. (Hebrews 5:1-10 ESV Emphasis Mine)