And you should bring near to yourself your brother Aaron, and his sons with him, from among Bnei Yisrael to serve Me [as Kohanim] Aaron, Nadav, and Avihu, Eleazar, and Itamar, Aharon's sons from the midst of Bnei Yisroel to serve me. (Shemos 28/1)
The Medrash (Medrash Rabah 35/7) explains that the possuk (Tehillim 119/92); "lulay soroshsecha sha'ashu'iy ahz ahvadahti b'awnyi- If Your Torah would not have been my pleasure, I would have perished in my suffering" is referring to our possuk when Hashem told Moshe to appoint Aharon and his sons to be Cohanim. The Medrash explains When HaKadosh Baruch Hu said to Moshe, 'And you should bring close to you Aharon... to serve Me...' Moshe was distressed. Hashem said to Moshe, 'I had the Torah and I gave it to you. If not for this, that I gave the Torah to you, the whole world would have been destroyed.'"
Zera Shimshon asks, when Bnei Yisroel were still in Mitzrayim and Aharon heard that Moshe was appointed by Hashem to be the leader of Bnei Yisroel, Aharon was very happy for him. How, then, can we understand that Moshe, who was the world's humblest person, was upset when Aharon and his children were picked to be the Kohanim instead of him?
Secondly, how is the possuk .... An appeasement?
Zera Shimshon explains the Medrash in the following way.
It is written in the Gemara that initially Moshe was supposed to be the Kohen Gadol and not Aharon. However, at the S'neh- the Burning Bush- Moshe was reluctant to be the leader of Bnei Yisroel and as a punishment for this Hashem took away the priesthood from him and Hashem gave it to Aharon.
It is written in the Medrash (Yalkut Shemoni Zechariah remez 577) that the redemptions of Clal Yisroel were never complete and they are always followed by new exiles. The reason for this, writes the Medrash, is because they were carried out by people. The only redemption that will be final and everlasting will be the one when Hashem Himself will be the redeemer with no intermediary.
According to this, explains Zera Shimshon, we can understand the give and take between Hashem and Moshe at the s'neh. Moshe argued that it would be detrimental for Klal Yisroel for HIM or any person to redeem them! Moshe wanted Hashem to take Bnei Yisroel out of Mitzrayim in order for this to be the final redemption.
Hashem, on the other hand, obviously knew at the time of the S'neh that Bnei Yisroel were going to sin with the Golden Calf and since the purpose of exile is to purify us from our misdeeds, Hashem didn't want to be our direct redeemer from Mitzrayim for the simple reason that if Hashem would have redeemed us there would be no subsequent exiles. This means that we would never be fully purified from the future sin of the Golden Calf and subsequently we would never be able to experience maximum closeness to Hashem. Therefore, Hashem ignored or refused Moshe's request and Hashem stayed with the appointment of Moshe.
In light of the above, we can now explain the Medrash.
When Hashem told Moshe to appoint Aharon and his family to be the Kohanim, Moshe was distressed. It wasn't because Aharon and family were appointed instead of him. Moshe who was the humblest person in the world was very happy for his brother just like Aharon was happy when he learnt that Moshe will be the redeemer of Bnei Yisroel from Egypt. The reason for Moshe's distress was that since the appointment of Aharon took place AFTER Bnei Yisroel sinned at the Golden Calf, Moshe was upset with HIMSELF that many years previously, at the s'neh, he didn't immediately accept Hashem appointing him to be the redeemer of Bnei Yisroel. He felt that he might have indirectly caused the incident of the Golden Calf because if he had immediately accepted to be the redeemer, without hesitation, the merit of this might have prevented that whole incident!
The reason that he thought of this at that time was because Aharon merited to become the Kohen Gadol at the s'neh, when Moshe hesitated to be the redeemer. Therefore, he was reminded of the incident of the s'neh when he was actually consecrated Aharon for that position.
Hashem noticed Moshe's reaction and Hashem told him, 'I had the Torah and I gave it to you. If not for this, that I gave the Torah to you, the whole world would have been destroyed.' Meaning, Hashem was not appeasing Moshe, but rather he was agreeing with his distress. Hashem only asked him, "Why only now?". Hashem told Moshe that he should have realized that Bnei Yisroel were going to sin in the future at the time when Hashem gave the Torah through Moshe and not directly from Hashem.
How is this?
Chazal teach us although the Yetzer Hara completely vanished when we heard the first two commandments from Hashem, it returned when Bnei Yisroel asked that Moshe tell them the rest of the mitzvos.
Therefore, Moshe should have thought and realized at that time that the reason that Hashem didn't give the Torah directly to Bnei Yisroel but through Moshe was because Hashem knew that Bnei Yisroel were going to sin in the future and if Hashem would have given the Torah directly to Bnei Yisroel and therefore they would not have had a Yetzer Hara, they would not have been able to excuse what they did with the familiar excuse of "the Yetzer Hara made me do it!". This would result in their being totally annihilated. Therefore, at that time Moshe should have already been distressed that he hesitated to accept the position to redeem Bnei Yisroel and that he indirectly brought about Bnei Yisroel's sin of the Golden Calf.
In short; Zera Shimshon quotes a Medrash that at the time that Hashem told Moshe to consecrate Aharon and sons to be Kohanim, Moshe was distressed. Zera Shimshon asks, why should Moshe, who was the humblest person ever to live, feel bad that his brother will be he Kohen Gadol and not him?
Zera Shimshon answers that that is not the reason that Moshe was distressed. He was distressed that at the s'neh, the Burning Bush, he hesitated to redeem Bnei Yisroel because he wanted Hashem to be the redeemer so there would be no subsequent exiles. He was afraid that his hesitation indirectly caused Bnei Yisroel to sin at the Golden Calf.
Hashem agreed with Moshe but asked that he should have realized his mistake at the s'neh and that Bnei Yisroel were going to sin and need more exiles to purify them, by the fact that Hashem gave the Torah through Moshe and not directly.
