What is the connection between Moshe's sin and his punishment? Furthermore, according to the Rambam's explanation, why was Moses angry when God was not?
Moshe committed a sin, because of which he was not allowed to enter the land of Israel. The text of the Torah is unclear as to what exactly was the sin that he committed. If his transgression was that he hit the rock, as Rashi understands, then why did God command him to take the rod? It would seem that God's intention was that he should use the rod also.
The Rambam claims, in the fourth chapter of his introduction to Pirkei Avot, that Moshe's sin was that he got angry. He spoke to the people in a furious manner, which was particularly improper for one whose behavior would be watched and emulated. Furthermore, by behaving in an angry manner he gave the impression to the people that God was angry with them also. This was incorrect. While God, on other occasions, is described anthropomorphically as being enraged, here He simply commanded Moses to perform this miracle. The fact that the people behaved in an aggressive manner towards Moshe (Bamidbar 20:3) does not mean that their behavior unleashed God's wrath. In showing anger, Moses gave the impression that it did.
For the Rambam this is a crucial issue. Firstly, the Rambam rules (Hilkhot Teshuva 7:3) that one must repent over bad character traits, just as one must repent bad deeds. Secondly, he claims that one who is not of a virtuous and exemplary character cannot possibly prophesy. This does not mean that a prophet must be perfect, as this episode concerning Moshe proves. However, a prophet is not simply God's mouthpiece, but an image to be emulated. Therefore, the way he presents himself to the people is of the utmost importance.
What is the connection between Moshe's sin and his punishment? A divine punishment is not merely a technique for putting one to order. There is always, we assume, a direct connection between the nature of the sin and the form of punishment. Furthermore, according to the Rambam's explanation, why was Moses angry when God was not?
When, at Mount Sinai, the people asked that they cease to experience the direct encounter with God, Moses became upset, and was calmed by God. Moshe wanted the people to experience the ultimate; God, however, knows human weakness. Moshe led the people through the wilderness for forty years, while they lived a miraculous existence. Now, on the verge of entering the land, they complained bitterly about the manna, the source of this miraculous existence. Moshe could not take it, and he lost his temper. As a spiritual leader, he was unable to deal with the idea that people lose interest in miraculous sustenance, just as he was upset by the people's withdrawal at Mount Sinai. God, however, understands. Therefore, both here and at Sinai there was a discrepancy between God's reaction and Moshe's.
The people were on the verge of entering the land and beginning to lead a natural existence. Moshe showed here that he was unable to deal with the people's succumbing to mediocrity. He demanded from them an ideal form of spiritual life. God also demands this of us; but He understands when we fall, and sees the difference between various forms of weakness. Moshe was let down. In his anger, he revealed that he was simply unable to make the necessary move between being the leader in the wilderness, and leading the people into a natural existence in the land of Israel. His sin revealed an inability to perform the mission that he was deprived of, as a punishment for his sin.
Courtesy of Yeshivat Har Etzion - www.etzion.org.il