“And Moshe said to Aharon, ‘Approach the Altar and perform your Sin Offering and your Elevation Offering, and offer atonement for yourself and for the people, and offer the sacrifice of the people, and atone for them, as God has commanded.'” [9:7]
Why does Moshe need to tell Aharon, “approach the Altar?” Rashi explains (from the Medrash) that Aharon was embarrassed, and was afraid to approach it. Moshe said to him, “Why are you afraid? You were chosen for this.”
The Medrash says that the Altar “looked like the Golden Calf” to Aharon, which is what caused him to be afraid.
The Minchah Belulah explains: as we all know, our minds play tricks on us, causing us to “see” things which are not really there. What our minds cause us to perceive are things which are in our minds in any case, in our subconscious and conscious thoughts. What was constantly on Aharon’s mind? The Golden Calf. He remembered his sin constantly.
What was his sin? He made the Calf. But the Medrash explains to us that Aharon witnessed the murder of his nephew Chur, when the latter rebuked the nation for contemplating a premature replacement for Moshe. Aharon realized if they killed him, the Kohen, as well, the sin would be unbearable. “Better,” he said, “that the sin be on my shoulders.” [Rashi to Exodus 32:5] By our standards, his “sin” was small indeed.
Yet he recalled it constantly. He fulfilled the verse in Psalms [51:5] “And my sins are before me always.” Even when he looked upon the Golden Altar, he was reminded of the Golden Calf.
This, says the Minchah Belulah, is why Moshe said “You were chosen _for_ this_.” For this very reason, that you constantly recall your sin, and are embarrassed for it, this is why you were chosen to be the Kohen Gadol.
The Evil Inclination uses many different methods to thwart people who want to come closer to G-d. One favorite is to cause a person to think of all his sins, and cause him to think that he’s worthless, and not worthy of attempting to come close.
Moshe taught his brother, being concerned about your past behavior is a _good_ thing. It means that you really care about the bad things which you have done. One can only become a “bad person” by imagining that his evil is good, by thinking that his behavior is acceptable. So don’t let your thoughts drag you down, he said. They aren’t a reason to turn back — through your regret, you are worthy of coming close!