The book HaYom Yom, from Day to Day, daily is a source of deep insights that I find very profound...from Thursday, 27 Adar II, 5765 - April 7, 2005, a very complex entry explaining why Aharon's son's died but a level we should still strive for...
(The 6th grand rabbi of Lubavitch hassidim notes this entry is from the collected holy writings of his father, the 5th grand rabbi...)
From my father's Sichot (collected talks):
The sin of Aharon's sons was "...when they drew close before G-d and died."
This was Ratzo without Shov, return.
The truth is that when coming "before G-d" there must be Tit'haru, one must be clean and pure, and (the closeness) must be evident in actual deeds. That which is on the very highest planes descends to the very lowest depths; hence Ratzo without Shov is death.
Continuing, G-d commands Moshe (Vayikra 16:2) to speak to Aharon.
The letters in the name Aharon are those in the word Nir'eh, "visible," which in terms of the powers of the soul refers to intellect.
Moshe is to tell Aharon that in order to enter the sacred (area), even "within the curtain" - meaning the state prior to tzimtzum ["Contraction" and concealment of G-d] - he must know that... upon the ark (Ha'aron - the letters of this word spelling nir'eh, "visible" - i.e. intellect) there is a Kaporet, a covering, an inner intent and purpose of a concealment, expressed in the words, "the face of (or "the inner aspect of") the Kaporet."
For this reason one "should not die," i.e. he should not stop with Ratzo, the entering alone - for "...in a cloud will I appear on the Kaporet;" [a] the inner intent and purpose of the first tzimtzum is revelation.
The introductory word to all this is V'al, "he shall not (enter)."
This expression of negation [b] indicates total nullification of self, Bitul, doing what Chassidus demands, not what his intelligence dictates. Then "he shall enter the sacred (area)."
a. Paradoxically, it is only through G-d's concealment ("...in a cloud...") that He is accessible ("..will I appear...").
b. Man's natural preference is what he feels is satisfying. Rejecting what he "knows" to be desirable and fulfilling - in favor of what Chassidus explains as the wish of G-d, is an act of self-negation. This prepares him to enter the Holy of Holies.
BS"D - בסיעתא דשמיא