No one takes this honor upon himself but only when called by God, just as Aaron was. In the same way, it was not Christ who glorified himself in becoming high priest.
While St. Paul, in his Letter to the Hebrews, generally distinguishes our Savior from Aaron (showing that the priesthood of Christ is greater than that of Aaron), in this place the Apostle emphasizes this point of similarity between the Levitical priesthood and the eternal priesthood of Jesus.
Namely, St. Paul tells us that, just as Aaron was called and ordained a priest, so too was our Lord. For no man can be a high priest of himself, but only when he is so called by God.
When, we ask, was Jesus called and ordained to the priesthood? When did he become a priest?
The answer here.