The Dubno Maggid told a story of a royal official whose son committed many crimes. The king was very fond of the official and because of his supplications the son was pardoned. Upon release, the son immediately resumed his wicked activities. This deeply saddened the father who realized that his son was taking advantage of his high position and relying on that to escape punishment. The official, therefore, asked the king to accept his resignation, so that his son could no longer rely on his protection.
This was the case with the Jewish People's reliance on their leaders. They murmured and rebelled against Hashem's leadership while expecting the merits of Moshe and Aharon to help them escape His wrath. Moshe came to the point of offering to step down as the leader, in the hope that the people would then see the consequences of their errors and correct their behavior. Moshe's leadership was coming to a close and Moshe reminded Bnai Yisrael of all the different stages of travel they had gone through under his leadership. This was intended as a warning to the Jews that they could no longer rely upon Moshe's merits to carry them through, but they would now have to accumulate their own merits and pay the consequences of their actions.