Hashem said to Moshe: VE’ATAH HAKREV ELECHA ET AHARON ACHICHA, “And you bring Aharon your brother close to you…”. (Ex. 28,1) The purpose was to inaugurate Aharon into the position of Kohen Gadol. The Midrash tells us that when this happened Moshe was distressed. (Shemot Rabbah 37,4). He wanted the position for himself but Hashem said to him “I gave you the Torah”, implying that that is a greater honor.
The Dubnow Maggid explained it by comparing it to a suit of clothes. The suit has to fit. It cannot be worn by someone too small or someone too big. Similarly the leadership of the Kohen Gadol has to fit the people. Moshe was too great a personality and could not properly perform the functions required by this leadership.
The Talmud states also YIFTACH BEDORO KESHMUEL BEDORO, “Yiftach in his generation is like Samuel in his generation”. (R.H. 25b) Yiftach was a plain individual and he became the leader of the Jewish people in his time because he successfully defeated Israel’s enemy. Shmuel was a great prophet and was leader of Israel in his generation. The Talmud stresses the belief that every generation has the kind of leader that that generation deserves. Hence the leader must be respected in his time.
This is what Hashem said to Moshe. You are too far above the level of the people. Aharon fits the position better than you.