Rashi wonders why Moses needed to tell this to his brother. Surely Aharon knew what his duty was!
But Rashi explains, “Aharon was hesitant, so Moses had to say to him, ‘Why are you hesitant? This is what you were chosen for!'”.
Rabbi Moshe Leib of Sassov offers an amazing comment on the commentary.
Why, he asks, does Moses tell Aharon, “This is what you were chosen for?” Most of us would interpret Rashi’s words like this: “Go, approach the altar, for this is what God intended!”
Rabbi Moshe Leib understands Moses differently: “Aharon,” he believes Moses was saying, “So you are diffident? Fine. That is how God made you. Diffidence, modesty, humility – they are part of the make-up God gave you. You are diffident because that is what God wants. It is far better than being too conceited and over-confident.
“But you have to overcome the diffidence and stride ahead and do your duty regardless, and God will be with you.”