Shemot Rabbah 43:3
The rabbis asked, "What does vayachal Moshe mean?" He made that which was bitter, sweet. Vayachal is language of sweetness. How is that? Rabbi Berechya in the name of R. Chiya bar Ada from Yafo in the name of R. Shmuel bar Nachman: when Israel came to Marah what is written there, (Shemot 15:23): "And when they came to Marah" Moshe began to criticize himself saying, "Why were these waters even created? What is their use in the world? It would have been better had they not been created." God knew what he was thinking in his heart, and so the Holy One Blessed be He said, "Don't say this, for are they not the work of my hand? Is there anything in the world that was created for no purpose? Rather, I will teach you what to say instead. Say this: Make the bitter, sweet." And from where do we know that the KBH said this? See what is written there: (Shemot 15:25) "Vayitzak Adonai v'yorayhu Adonai etz." And he cried to God . "Vayorayhu" tree. It is not written "vayAhrayhu," rather it is written "VayOrayhu." The language "VaYOrayhu" is the language of teaching, as it says (Mishlei 4:4) “God instructed me and said to me”—and it is written (Shemot 35:34) “and giving instructions, he put in his heart..”
So Moshe had learned something extremly profound, but he did not use it. And when did he use it? When Israel came to the desert and God wanted to destroy them, Moshe said to him, “Master of the universe, you wish to banish Israel, you want to destroy them from the world. But didn’t you teach me at Marah: that I should pray and say, ‘Make the bitter sweet.’ So now, sweeten the bitterness of Israel and heal them.” That is the meaning of Vayachal Moshe.
Why this text is beautiful: There is something wonderful about God insisting that even the things that seem to have no value in the world have value and that we shouldn't give up on anything. There is also something amazing about the way that Moshe is so upset about the waters-- and the way that the text says that "Moshe began to criticize himself" when the waters seemed bitter. It was as if he was giving up on himself and not just on the waters. God assures Moshe that everything would work out in the end.
And then, Moshe is able to remind God what God had taught him when God wanted to destroy the Israelites in the desert is just amazing. It says a lot about the truth of the goal of Judaism being to help make the world full of sweetness. And it says a lot about God that God would let his will be quieted by this reminder by Moshe.
Finally, it reminds us that trees should be used as a symbol that part of what we need to do in the world is find the good in everything, for it was a tree that Moshe threw into the water to make it drinkable for the Israelites.
