Rebbe Bunim of Peshischa zt'l (quoted in Sifsei Tzaddik, Korach 8) explains that Korach was upset that he couldn't daven in the Kodesh HaKadoshim like Aharon HaKohen. He didn't know that one can attain the same perfect tefillah wherever he is.
The Gemara (Pesachim 50.) tells that Rav Yosef, the son of Reb Yehoshua Ben Levi, was niftar and then returned to life. His father asked him what he saw in heaven. He replied, "I saw an upside-down world. Those on top here are at the bottom there, and vice versa." His father corrected him, ראית ברור עולם, "You saw a true world." Rashi explains that he was referring to the wealthy. They are at the top of our world, and people honor the wealthy. But they may be at the bottom in heaven because wealth isn't what makes someone honored in heaven, only Torah and mitzvos.
The Mabit (introduction to Beis Elokim) disagrees with this explanation because Rav Yosef wouldn’t have been surprised by seeing that only Torah study and mitzvos count in heaven. Therefore, the Mabit explains that Rav Yosef was referring to ovdei Hashem, people who serve Hashem. There are ovdei Hashem who are very respected in this world but less so in Olam HaBa and visa-versa, because there are people who we think are mediocre or even less than that in their spiritual practices, but they are extremely honored in heaven. He discovered that things aren't always the way they seem.
This was Korach's error. He thought he must have the most prestigious position in this world to earn the highest place in Gan Eden, but it isn’t so.
The Gemara (Rosh Hashanah 11.) states, "Everything in creation was created according to their agreement and consent." The Arugas HaBosem (Chayei Sarah) writes that this means that the earth also agreed to be created in its shape and form. Compare the coarse earth to the divinity of heaven, and we struggle to understand why indeed the earth would agree to be created in its lowly form.
The answer is that the earth also has a mission in the world, and its mission isn't less important than that of the heavenly creations. Indeed, it is the tests that Hashem presents to mankind who live on and are drawn to earthly desires that make life worthwhile and give meaning to our avodas Hashem. The earth understood that and agreed to be created in this manner. It is called ארץ, from the word רצה, want, because the earth wanted to do Hashem's will and it wanted to be created this way.
The Arugas HaBosem explains that no one should complain, "Why can't I be like that person? If I were like him, with a sharper mind and brought up under better circumstances, I would also be a genius in Torah!" Don't say that, because Hashem created you the way you are because this is the only way you can reach perfection."
It states (Tehillim 101:6), ארץ בנאמני עיני. The Arugas HaBosem explains that Hashem's eyes are with those who have emunah like the earth and are prepared to do Hashem's will, even when it seems that there are more prestigious services to perform for Hashem.
Korach was on a high spiritual level. As a Levi, Korach was bestowed with the honor of carrying the aron. Arizal taught that in the era of techiyas hameisim, Korach will be the kohen gadol. The Chidushei HaRim zt'l says that if it weren't for this machlokes, Korach would have become the levi gadol — an extremely great honor that no one has ever received so far.
But he wanted more. He wanted to serve Hashem in the most perfect way. His error was that he failed to understand that, with the portion Hashem gave him, he had the potential to be the greatest tzaddik.
The Arugas HaBosem explains that the earth swallowed Korach because it was the earth that understood that one should only desire and focus on the task Hashem set out for him.
