After Aaron dies, the verse states that “the entire house of Israel” cried over him. According to the Sages, Aaron would facilitate peace between people; that is why he was so beloved.

[The mishna writes that one should act like Aaron, and be a] “lover of peace.” How so? This teaches that a person should love and facilitate peace in Israel among all, just as Aaron would love and facilitate peace in Israel among all. As it is stated: “The Torah of truth was in his mouth, and injustice was not found on his lips; he walked with Me in peace and honesty and he returned many from iniquity” (Malachi 2:6). Rabbi Meir says: What is the meaning of the verse: “And he returned many from iniquity”? When Aaron would proceed along the way and he encountered a bad or wicked person, Aaron would greet him. The next day, when that man would seek to violate a prohibition, he would say: Woe is me; how will I later be able to look Aaron in the eye if I do this? I would be embarrassed, after he greeted me. The result is that this person would prevent himself from transgressing. Moreover, when two people would engage in a quarrel with each other, Aaron would go and sit with one of them, and would say to him: My son, see your fellow. What is he saying? He is sorrowful and rending his garments. He is saying: Woe is me; how will I be able to look my fellow in the eye? I am embarrassed before him, because it is I who wronged him. Aaron would sit with the first person until he removed all the enmity from his heart. Aaron would then go and sit with the other, and would say to him: My son, see your fellow. What is he saying? He is sorrowful and rending his garments. He is saying: Woe is me; how will I be able to look my fellow in the eye? I am embarrassed before him, because it is I who wronged him. Aaron would sit with the second person until he removed all the enmity from his heart. When the two individuals would meet each other, they would hug and kiss one another. Therefore, it is stated: “They wept for Aaron thirty days, the entire house of Israel” (Numbers 20:29). (Avot deRabbi Natan, version A, chap. 12)