Rashi says, “Aaron was diffident and was afraid to go forward; Moses said, ‘Why are you diffident? For this you have been chosen!”‘
What held Aaron back? The sages say he saw the altar in the shape of an ox and was frightened by it. The task was massive and he was clearly daunted.
Another possibility is that his conscience was bothering him. He, after all, had been the one to encourage the people to build the golden calf. Can a man who once condoned idolatry now serve God? One presumes he had thoroughly repented; otherwise God would not have deemed him worthy of office.
But how heavily should a leader’s past misdeeds weigh? Should they prevent him from being rehabilitated?
The rabbis say that a leader should in fact be a person with a basket of snakes on his back. Even leaders make mistakes; the question is whether they are honest and courageous enough to admit they have been wrong and to be doubly careful in future.