At the dramatic scene of the burning bush, God appoints Moses to lead the Israelites out of their bondage in Egypt. Moses is reluctant to accept this responsibility and asks God to choose someone else. He claims that he is not articulate enough, perhaps reflecting a more general feeling that he was not up to the task.
God insists that Moses take on this responsibility. He tells Moses that his brother Aaron will be at his side, and will be able to speak on behalf of Moses. God informs Moses that Aaron will come to meet him, "vera-akha, vesamah belibo", and he will see you and rejoice in his heart. These three Hebrew words have tremendous meaning, and tell us much about the greatness of Aaron and why he became the beloved High Priest of the people of Israel.
Aaron was older than Moses. Aaron had been living in Egypt all these years when Moses was living in peace as a shepherd in Midian. Aaron had to deal firsthand with the slavery of his people, and obviously had a much clearer understanding of the situation than did Moses. One might have thought that Aaron was more entitled to have been chosen by God to be leader; he was older, more experienced, and more directly involved with the people of Israel. And yet, God chose Moses!
How would we imagine Aaron's reaction upon learning that God had chosen his younger brother, a shepherd in Midian, to be leader of Israel? We might have expected that Aaron would be jealous, angry, insulted, resentful. But God tells Moses: Aaron will see you and rejoice in his heart! Not only was Aaron not upset, but he genuinely rejoiced in Moses' success. Aaron was not an egotist, he was content with his lot. He was not just superficially courteous to Moses, but he "rejoiced in his heart", sincerely and totally. Aaron had a unique capacity: the capacity to love, to rejoice fully in the success of others without feeling a grain of jealousy or ill-will.
It is not easy for people to rejoice in the success of others. People think: I should have received that honor, I am more deserving, I am more qualified. It is not easy for people to rise above egotism, jealousy, resentfulness. To do this requires tremendous self-confidence, spiritual poise, serenity--and love. It requires the ability to transcend one's own ego, and celebrate in the virtues and successes of others. Aaron had these virtues.
The Pirkei Avot describes Aaron as a person who loved peace and pursued peace, who loved his fellow human beings and brought them closer to Torah. He gained this reputation because he genuinely was able to care about the needs and feelings of others, without insisting that he receive honors and accolades. He was a loving person--and therefore he was beloved.