The Meshech Chochmah, R' Meir Simcha of Dvinsk zt”l writes that Hashem commanded Moshe to anoint the sons of Ahron, to signify the essence and spirit in which he was to perform the ceremony.

     Moshe was not jealous of the כהונה - priesthood, that had been conferred upon his brother Ahron, because he himself was the Navi and king of his people and had even fulfilled the functions of the Kohen Gadol during the seven days of preparation before Mattan Torah. But Moshe might well have resented the fact that his own children could not have been raised to this lofty position.

     The Medrash tells us that Moshe Rabbeinu asked Hashem to pass on his own qualities to his children, but Hashem refused his request. It was for this reason that Hashem reminded Moshe that when he was to anoint his brother's sons, he was to do so with the same joy and eagerness as he had shown when consecrating their father.