Rashi says that Moshe had to order Aharon to approach the Mizbeach because Aharon was bashful and afraid to approach. Moshe said to him: “Why are you ashamed? For this you have been chosen!”
Sefer Degel Machaneh Efraim asks in the name of his grandfather, the Baal Shem Tov zy”a, how Aharon was convinced by Moshe’s words. How did being told that he was chosen for this job make him less bashful?
He answers that Moshe was telling Aharon that he was chosen for this job because he was bashful and afraid to approach Hashem. Hashem desires a humble person, and that is why He chose Aharon to be Kohen Gadol. Thus, Moshe told him that his trait of bashfulness was the reason he was appointed to be Kohen Gadol and only one with this trait could do this job.
