קַח אֶת-הַמַּטֶּה, וְהַקְהֵל אֶת-הָעֵדָה אַתָּה וְאַהֲרֹן אָחִיךָ, וְדִבַּרְתֶּם אֶל-הַסֶּלַע לְעֵינֵיהֶם, וְנָתַן מֵימָיו; וְהוֹצֵאתָ לָהֶם מַיִם מִן-הַסֶּלַע, וְהִשְׁקִיתָ אֶת-הָעֵדָה וְאֶת-בְּעִירָם
Take the rod, and assemble the congregation, thou, and Aaron thy brother, and speak ye unto the rock before their eyes, that it give forth its water; and thou shalt bring forth to them water out of the rock; so thou shalt give the congregation and their cattle drink. -Numbers 20:8
Instead of speaking to the rock, Moses decides to smack it twice, and as a result, he is denied entry to Israel. The question remains: why? Why would hitting a rock twice cause Moses' hard work of nearly forty years to be for naught because of a rock? The beauty of this answer, or rather answers, is that this becomes a nice prooftext of how the Torah is a multi-faceted text that speaks to man in more ways than one. With that in mind, let's find out why G-d denied Moses access to the Holy Land:
1) Take a look at the verse again, and you will notice that G-d told Moses to speak to the rock, not to hit the rock twice. According to Beer Mayim Chayim, the rock was not in its normal place, and Moses lost his patience. Therefore, he found another rock and did what he was used to doing (Exodus 17:6)--he hit the rock. For Maimonides, he thought that Moses had deviated from the golden mean of patience at this point. Not only do we see that patience is a virtue, but also have G-d emphasizing the character trait of bitachon, or trust in G-d. Moses' lack of [unwavering] trust is what caused it.
2) If you take a look at verse 10, you will see that it says: “Listen, O rebels, shall we bring forth water from the rock for you?” We can thank Ramban for this one. You give credit where credit is due, right? Moses apparently forgot that. In the previous miracles, Moses made it clear that G-d was the source of these miracles. This time, Moses did not make that clear. Moses forgot that G-d is the Master of the Universe and the ultimate source of everything. Because Moses did not sanctify G-d's name in front of the people Israel, he did not merit entry into Israel.
3) This insight comes from Rashi: anger management issues. Moses had to deal with a bunch of kvetchers for a long time. I'm surprised he didn't lose it earlier. But at the same time, if you lose your temper over something as simple as water, you know it’s time to appoint a new leader. Even for one moment, why would anger deny him the very thing he wanted? Because, as the Talmud points out in Shabbat 105b, anyone who tears his clothing, breaks vessels, or scatters his money in anger is an idolater. Why is anger akin to idolatry? Anger is about the self. It is about “me, me, me,” and subsequently takes G-d out of the picture because idolatry is, by definition, making anything but G-d the main focus. And you thought that idolatry was just about statues. Plus, when you’re angry, you don’t have any self-control. Without self-control, you’re no better than an animal—a concept we’ll touch upon in a moment. What can be imparted upon us is that even if justified in our anger, we should not always act on it just because we can. The fact that Moses could not exhibit self-control, a feature that separates man from animal, kept him from entering the land of Israel.
4) Rashi brings up yet another explanation. Since a rock has no free will, having an inanimate object “listen” to Moses means that the Israelites should do so all the more so. But this effect was lost when Moses hit the rock.
5) I have a fifth interpretation to add, one I came upon very recently. This one takes a little closer look at the Hebrew text itself. When G-d tells Moses to speak to the rock in verse 8, G-d tells Moses to give drink to the assembly and the animals (בְּעִירָם). In this verse, the direct object indicator את is used. However, look in verse 4 (in the national grievance) and 11 (when they receive the water). The absence of the את in verses 4 and 11 shows us something very important—since they did not use the את to distinguish between man and animal, they viewed themselves as animals (Meshech Chochmah). They were acting on pure animal instinct, which, according to Maslow’s Hierarchy of Needs, is on the bottom. The people were acting like animals. Where I surmise that this animal behavior is linked to Moses' denial to the land of Israel is when you see two verses next to each other, you can derive interconnectedness, much like Rashi did with the sotah and Nazirite. With that in mind, we see that Moses didn’t correct this misperception, and as such, he was not allowed in the Land of Israel.
Which reading of this text is correct, you might ask? Since they all have textual backing, the answer is "all of them!" In a nutshell, 'tis the beauty of Judaism. This is so fascinating to me that I had to publish this, even though we technically just began Parshat Balak. Impatience doesn't exclude the anger. Anger doesn't exclude a reading on what it means to be human or constantly thanking G-d for providing us with our needs. As a matter of fact, most of these responses are intertwined.
Again, a theme which I will use again and again--truth speaks to us in many ways. It's not that absolute truth doesn't exist. It's just that reality, and subsequently truth, are more complicated and nuanced than one can imagine. That's why I love Judaism in its authentic form. We can grapple with the many facets of truth while recognizing the source of that reality.