ותצא אש מלפני יקוק ותאכל אותם וימתו לפני יקוק
A fire went forth from before Hashem, and consumed [Nadav and Avihu], and they died before Hashem
At the conclusion of the Mishkan’s inauguration, the people were beset with tragedy. Nadav and Avihu, two of Aharon’s children, who were leaders of the Jewish people, died. They had volunteered an unrequested fire offering to Hashem, and perished instantly. The consequences of their actions seem too severe for their “crime”. Indeed, what they did seems meritorious. They were displaying their devotion to Hashem, and their joy at the opportunity to express it. As a result, many explanations are given for what their real crime was.
One explanation involves a background story not detailed by the Torah. Moshe and Aharon were walking, with Nadav and Avihu following, and the rest of the generation were behind them. Nadav said to his brother Avihu: “When will these two elders pass away, and you and I will become the leaders of this generation?” Hashem said to them: “We shall see who will bury whom”. From a cursory glance at this story it would seem that Nadav and Avihu’s rude and inappropriate remarks and thoughts are what caused their early demise. But this can’t be the explanation. Nadav and Avihu were among the greatest of the generation. What’s really going on here?
If we take the whole story in context, we might gleam the proper understanding of what the real problem was with Nadav and Avihu. A different verse which describes their death says: בקרבתם לפני יקוק וימותו, with their coming close before Hashem, they died. Why does the Torah stress that they came close? We see from here their very intense desire to connect with their creator. Another explanation given for why they died is that they came too close to Hashem by entering the Holy of Holies. A different one is that gazed intently at the Divine Presence like one would a friend. We don’t have to say that these explanations disagree. They’re all painting the same picture: Nadav and Avihu had a very intense desire to come as close as possible to Hashem.
We are taught that the leader of the generation is only as great as the people to which they lead. We see this with Moshe. When the Jewish people sinned with the Golden Calf, Hashem told him to descend from his prominence, as his people had fallen. The opposite is also true: when the people are elevated in stature, their leader is similarly elevated. Now, the generation that left Egypt was one of the loftiest of generations. They merited to a Divine Revelation at Mount Sinai. They witnessed all the miracles of the Exodus, as well as many more throughout their years in the wilderness. They had an intimate knowledge of Hashem. No generation will be as lofty as that one until the times of Moshiach.
Now we can have a better understanding where Nadav and Avihu were coming from. As stated, they had a very intense yearning to be as close as possible to Hashem. As they were walking behind Moshe and Aharon, with the rest of the generation walking behind them, they started to contemplate. They realized just how precious this generation was, and how close they were to Hashem. They knew there would be no generation like it. They subsequently had an enormous desire to merit to be the leaders of that specific generation. Since the leader is as great as its followers, they knew it would bring them to greater levels of closeness to Hashem.
When they asked, “When will these elders die, and we will lead this generation?”, their intention was that specific generation. They weren’t intending to be disrespectful. They just had their sights set on that generation, the only one that would bring them the closest to Hashem. Even though they had the proper attitude, with their desire to be as close as possible to their Creator, nonetheless their comment was disrespectful. This wasn’t the way they should have spoken about Moshe and Aharon. As a result, they were shown no favoritism when they brought an alien fire into the Temple.
Despite their misdeed, we can learn from Nadav and Avihu the proper attitude to life. Although a person should always be satisfied with their lot, this refers only to their physical situation. A person should never be satisfied with their spiritual position, and should always be seeking greater heights in their connection to Hashem. That’s the praiseworthy trait of Nadav and Avihu, which is surely worth emulating.
 Based on Be’er Yosef to Leviticus 10:2
 Leviticus loc. cit.
 See http://parshaponders.com/shemini-5777 for Rav Dessler’s presentation of fifteen different explanations from Chazal and Rishonim
 Sanhedrin 52a; Toras Kohanim and Midrash Aggadah to Leviticus 10:1; Vayikra Rabbah 20:10; Midrash Tanchuma Acharei Mos § 6; Tanchuma Yashan Acharei Mos § 7
 Some of the above sources are coming from Exodus 24:1, which says the order of those who merit to ascend Mount Sinai: First Moshe, then his brother Aharon, then Nadav and Avihu (note: and not Aharon’s other two sons), then the seventy elders. This appears to show Nadav and Avihu’s prominence amongst the leaders of the generation
 See Torah Sheleimah to Exodus loc. Cit. § 5 for another explanation than that of the Be’er Yosef
 Leviticus 16:1
 Toras Kohanim ad. loc. Vayikra Rabbah 20:8; Midrash Tanchuma loc. cit.
 Vayikra Rabbah 20:10; Bamidbar Rabbah 2:25; Midrash Tanchuma and Tanchuma Yashan loc. cit. See Rashi to Exodus 24:10-11
 Berachos 32a (brought by Rashi to Exodus 32:7); Shemos Rabbah 42:2; Midrash Aggadah to Exodus 33:1; Pirkei D’Rabbi Eliezer Chapter 45 with Radal; Pesikta Rabbasi 10:6. See also Tanchuma Yashan Ki Sisa § 13
 See Toras Kohanim to Leviticus 27:34
 They are known as the דור דע, the generation that was full of knowledge. Sources for this expression include Vayikra Rabbah 9:1, Bamidbar Rabbah 19:3, Koheles Rabbah 7:23, Midrash Tanchuma Chukas § 6, Tanchuma Yashan Chukas § 13, Pesikta Rabbasi 14:1, Pesikta D’Rav Kahana 4:3
 Zohar III Shelach p. 168a
 “When they brought an alien fire into the Temple” is my addition to how the Be’er Yosef ends. He simply writes that they were shown no favoritism
 Avos 4:1
 Heard in the name of Rav Chaim Shmuelevitz