The Midrash tells us that Moshe sinned with the word “az” when he complained “U’mei’Az basi el Phraoh l’dabeir b’shemecha he’ra l’am hazeh,” and he corrected his sin with the use of the same word “az” when he sang “Az yashir Moshe.” It's not the coincidence of a common word which the Midrash is emphasizing, but rather the thematic relationship between these pesukim and events.
The Beis HaLevi’s explanation of the Midrash is one of the more famous/popular ones: At the time of redemption we will be privileged to look back and understand how all that we thought was bad and to our determent was really a bracha in disguise. Moshe sang “Az yashir” with the realization that the suffering he complained of and questioned, “m’az basi el Pharoah,” was really all for the good.
Shifting from Brisk to Telz, R’ Bloch offers a different explanation which I found amazingly deep and insightful, but at the same time I am troubled by the message. The Midrash elsewhere tells us that despite the many miracles G-d performed before the redemption from Egypt, no one ever sang shirah until Yam Suf. Why not? Why did Avraham not sing shira when he escaped from the kivshan ha’aish? Why did Yitzchak not sing shirah when spared from akeidah? Why didn’t Ya’akov sing shirah when he escaped Lavan’s home?
R’ Bloch explains that this lack of shirah is not due to some failing on the part of those tzadikim, but aderaba, precisely because of their greatness. Emunah means accepting with complete equanimity that all that occurs in the world is ultimately, whether we see it as such or not, a means to greater kvod shamayim and an expression of Hashem's perfect plan. Avraham accepted that whether he personally lived through the kivshan or not, the result will ultimately be greater kvod shamayim, so why sing shirah at being spared? Why sing shirah or be surprised at salvation when one knows that Hashem runs the world according to a plan which guarantees good for the righteous and evil for wrongdoers?
Moshe Rabeinu, however, did not accept with equanimity the suffering and pain of the Jewish people. He questioned and cried out with “az” because he could not see how kvod shamayim was enhanced by the prolonging of galus. And precisely because Moshe had the capacity to be moved and pained by Jewish suffering that gave him the capacity to be moved and rejoice in shirah when seeing the redemption of the Jewish people.
R’ Bloch contrasts Moshe’s shirah with the shirah of David haMelech. Chazal tell us a harp was suspended above David’s bed which played when the North wind blew. Unlike Moshe’s shirah which was inspired by the shift in circumstance from the “az” of galus to the “az” of geulah, David’s harp played in shirah even while he was still asleep and not conscious or aware of the comings and goings in the world; David’s shirah played as the North wind, the wind that always exists in nature, blew. David haMelech’s shirah was not inspired by the miraculous, but was a shirah that celebrated the day to day presence of Hashem in the world. This is the shirah which we will sing with the coming of Moshiach.
The interpretation is brilliant, but am uncomfortable with the upshot of the message. Who is superior – those tzadikim who accept with equanimity Hashem’s hashgacha, those tzadikim who do not suffer at the experience of pain or rejoice in redemption because it is so clear to them that all is in Hashem’s hands and the proper outcome is inevitable, or those tzadikim whose souls cry out at tragedy and soar in joy? R’ Bloch sees the former as having a superior level of emunah, while describing the latter having a emunah which is contingent (to some degree) on circumstance. Emunah which accepts all with equinimaity is superior to emunah which is diminished and challenged by the perception of evil, but which is elevated when salvation arrives.
I am uncomfortable with this idea of placing emunah is conflict with what I can only decribe as one's humanity. Need empathizing with suffering and tragedy, even questioning suffering and tragedy, be seen as indicative of a shortcoming of belief? Or might one not argue that it is precisely a deep-seated belief in hashgacha that leads one to protest suffering and question what appears to be injustice? Without belief, there is no room for questions, as an uncaring mechanistic universe does not promise either justice or sympathy.
Chazal tell us that Chizkiyahu would have been Moshiach had he only sung shirah when Sancheirev was defeated. The Ch. haRI”M explains (akin to R' Bloch) that it was the tzidkus of Chizkiyahu that led him to not sing shirah -- so confident was Chizkiyahu in his emunah that he too, like other tzadikim, accepted with equanimity the defeat of Sancheirev without shock or surprise. Yet, it seems to me that the punchline of the gemara is that singing shirah is the higher madreiga, as that would have led to the complete tikun of the world and the arrival of Moshiach. Yes, perhaps the ability to experience the joy of singing shirah comes at the price of feeling the challenge to emunah of pain and suffering, but sometimes the price is one worth paying.