Midrash Tanhuma Devarim 1:1
דבר אחר ישושום מדבר וציה מפני מה כתיב כך ללמדך שבשעה שהקב"ה מגלה שכינתו על ישראל אינו נגלה עליהם כאחת מפני שאינן יכולין לעמוד באותה טובה בפעם אחת שאם יגלה להם טובתו כאחת ימותו כלם...הקב"ה עאכ"ו אלא מה הקב"ה עושה מתגלה להם קמעא קמעא
"The wilderness and the parched land shall be glad, and the desert shall rejoice, and blossom as the rose" (Isaiah 35:1). Why was this verse written in such a descriptive manner? In order to teach that when The Holy One Blessed Be He revealed His presence to Israel, He did not reveal it to them all at once, because they would not have been able to stand the totality of God's goodness. For if God's goodness had been revealed all at once, all of Israel would have died . . . so what did the Holy One Blessed Be He do? He revealed himself to them bit by bit.
This week's midrash seeks to answer the question of why Moses needed to retell the entire Torah in the book of Devarim. Shouldn't one iteration of God's covenantal relationship with Israel have been enough? Using a particularly descriptive verse from Isaiah, our midrash's clear answer is that the repeated revelation of the Torah was necessary for Israel's sake. Had the full magnitude of the Torah, of God's presence been shown all at once, the Israelites would have been overwhelmed by the sheer awesomeness of the event, and would have died on the spot. So God disclosed the Torah and His presence bit by bit, over a period of time. Hence the need for the Devarim—to continue the process of revelation.
A valuable lesson lies at the heart of this midrash. When we are looking to create change in ourselves, our spiritual life, our community, and the world, the natural tendency is to want to do everything all at once—to achieve the instant gratification of rapid transformation. But like God imparting the entire Torah at once, this approach is not sustainable, and in fact can be harmful. Rather, gradual growth, learning, and slow transformation are the way to achieve true change, and to see God's full glory revealed in this world.