In the middle of Parshat Chukat, Aharon, the Kohen Gadol, passes away: “Vayivku Et Aharon Shiloshim Yom Kol Beit Yisrael,” “And all of Beit Yisrael mourned for Aharon for thirty days” (Bemidbar 20:29). In Sefer Devarim, when the Torah discusses Moshe’s death, the Pasuk (34:8) states only, “Vayivku Bnei Yisrael Et Moshe Be’Arvot Moav Sheloshim Yom,” “Bnei Yisrael cried for Moshe in Arvot Moav for thirty days.” There is no mention in the Pasuk regarding Moshe’s death of the entire nation weeping, as is recorded regarding Aharon’s death. Rashi explains that this is because Aharon was a Rodeif Shalom, a pursuer of peace. He established peace between a man and his friend and frequently brought harmony between husband and wife; hence, “The entire house of Israel” wept because Aharon’s passing was felt more by the common Jew. The Yalkut furthers this point, adding that Moshe’s job was to judge, and he therefore rebuked many people, which caused his truly pleasant personality to diminish in many people’s eyes.
The Or HaChayim also tries to resolve this difficulty. He quotes the Ibn Ezra who says that the outpouring of grief for Aharon showed respect for Moshe, his older brother, who survived to mourn him. The Chizkuni adds that Moshe wept first for Aharon, and when the nation saw Moshe cry, they too were moved to tears. Since Aharon’s death was sudden, the Or HaChaim continues, Bnei Yisrael had no time to prepare for his death. Moshe’s death, on the other hand, was forthcoming. Therefore, Bnei Yisrael were shocked when Aharon died and cried more than they did for Moshe’s death, with which they able to cope because they knew about it for a long time in advance.
Additionally, Aharon’s death caused the Ananei HaKavod to leave Bnei Yisrael, leaving Bnei Yisrael more vulnerable to enemy attacks. Although this also occurred following Moshe’s death, at that time, Bnei Yisrael were already looking forward to entering Eretz Yisrael, so their distress was not as heightened. Also, after Moshe passed away, Yehoshua was recognized as his successor. He did not fully take the place Moshe, but the mere realization that someone was taking over the position as leader was consoling. However, Aharon’s successor and son, Elazar, could not replicate Aharon’s warm personality and amicability towards the people, and Bnei Yisrael were therefore more upset by his death than by Moshe’s.
--Adapted from a Dvar Torah in Peninim on the Torah