Hebrews 5

Paul, being an ordained apostle and high priest himself wrote to the Jews -
"Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession." (New Testament | Hebrews 4:14)

In other words, as high priests, let us also magnify our calling -

"FOR every high priest taken from among men is ordained for men in things pertaining to God, that he may offer both gifts and sacrifices for sins..." (New Testament | Hebrews 5:1)

The Savior taught that after his resurrection we including high priests are to sacrifice, as the Savior taught -

"...ye shall offer for a sacrifice unto me a broken heart and a contrite spirit..." (Book of Mormon | 3 Nephi 9:20)

The Prophet Joseph F. Smith saw in revelation, high priests

"...who had offered sacrifice in the similitude of the great sacrifice of the Son of God, and had suffered tribulation in their Redeemer's name." (Doctrine and Covenants | Section 138:13)

Paul also teaches that those who are high priests -

"...art a priest for ever after the order of Melchisedec." (New Testament | Hebrews 5:6)

We learn in modern day -

"Why the first is called the Melchizedek Priesthood is because Melchizedek was such a great high priest.
3 Before his day it was called the Holy Priesthood, after the Order of the Son of God.
4 But out of respect or reverence to the name of the Supreme Being, to avoid the too frequent repetition of his name, they, the church, in ancient days, called that priesthood after Melchizedek, or the Melchizedek Priesthood." (Doctrine and Covenants | Section 107:2 - 4)

Paul teaches and important point regarding this priesthood -

"...no man taketh this honour unto himself, but he that is called of God, as was Aaron." (New Testament | Hebrews 5:4)

How was Aaron called and ordained? I would assume much like Joshua -

"And the LORD said unto Moses, Take thee Joshua the son of Nun, a man in whom is the spirit, and lay thine hand upon him;
19 And set him before Eleazar the priest, and before all the congregation; and give him a charge in their sight.
20 And thou shalt put some of thine honour upon him, that all the congregation of the children of Israel may be obedient.
21 And he shall stand before Eleazar the priest, who shall ask counsel for him after the judgment of Urim before the LORD: at his word shall they go out, and at his word they shall come in, both he, and all the children of Israel with him, even all the congregation.
22 And Moses did as the LORD commanded him: and he took Joshua, and set him before Eleazar the priest, and before all the congregation:
23 And he laid his hands upon him, and gave him a charge, as the LORD commanded by the hand of Moses.

(Old Testament | Numbers 27:18 - 23)

Paul even noted that of the Savior Himself -

"Though he were a Son, yet learned he obedience by the things which he suffered;
9 And being made perfect, he became the author of eternal salvation unto all them that obey him;
10 Called of God an high priest after the order of Melchisedec." (New Testament | Hebrews 5:8 - 10)

It becomes obvious that one just does not take this priesthood upon himself. It must come by one already having authority and authorized by one who hold priesthood keys to do so.