Understanding that he could not lead the Jewish people into Eretz Yisrael, Moshe pleaded with Hashem to allow him to give up his title as leader in exchange for the opportunity to simply enter Eretz Yisrael instead of dying outside its borders. The Midrash relates that Hashem agreed to abide by Moshe's wish so long as he, Moshe, found it tolerable. On the day Yehoshua was appointed leader, he entered the Ohel Moed, just as Moshe had done. All others, including Moshe, were kept out, just as had been the case when Moshe entered. When Yehoshua emerged, he was immediately approached by Moshe and asked what Hashem had revealed. Yehoshua responded that just as he was not privy to all that Moshe learned from Hashem in Ohel Moed, so too he in turn could not reveal to Moshe all that he was taught. When Moshe heard this response he cried out, "Better a thousand deaths than to suffer these pangs of jealousy."
Could it be that Moshe Rabeinu suffered from pangs of jealousy at seeing Yehoshua now taking his place? A Slabodka-ish answer would be that yes, indeed, even Moshe Rabeinu could suffer such pangs. No matter to what heights a person rises, he/she remains subject to human temptation and failings. If even a Moshe Rabeinu could be plagued by such concerns (albeit on his level, meaning he was attuned to the most fleeting feeling of jealousy which would not even register on our radar), how much more must we pay attention to our midos and remain attuned to possible lapses. Perhaps Moshe's recognition that he could not escape human failing brought with it the recognition that he could not escape mortality either.
R' Chaim Shmuelevitz, however, sees a deeper lesson here. Why couldn't Yehoshua have simply told his Rebbe. Moshe, what Hashem said? The answer is that communicating the message alone would not have been sufficient or satisfied Moshe. When Moshe or Yehoshua entered into the Ohel, or when a Navi enters into a prophetic state, it is not just about hearing words from Hashem, but it is an experience that envelops the entire person. Even if Yehoshua told Moshe what he heard, there is no way through words alone that he could capture the experience of nevuah and pass that on vicariously to Moshe.
We have discussed this idea many times in the past. Imagine trying to describe what a rainbow looks like to someone who is color blind, or trying to explain to someone deaf what a symphony sounds like. That's what trying to put Shabbos or talmud Torah into words is like. You have to hear Mozart and see Rembrandt -- listening with an attentive ear, observing with a discerning eye -- to appreciate their depth and beauty. One must immerse in the experience of Torah with an open heart and mind to gain what it has to offer; philosophical proofs alone cannot convey this appreciation.