The Parsha begins with the joyful celebrations of the Inauguration of the Mishkan (tabernacle), however this joyous occasion becomes a time of mourning with the sudden deaths of Aaron’s two oldest sons, Nadav and Avihu. "The sons of Aharon, Nadav and Avihu, each took his fire pan, they put fire in them and placed incense upon it; and they brought before Hashem an alien fire that He had not commanded them. A fire came forth from before Hashem and consumed them, and they died before Hashem."
Chazal offer a number of explanations as to the exact wrongdoing of these two great men which caused them to receive such a strict punishment. The Toras Kohanim writes: “...Nadav and Avihu did not seek advice from Moshe.. and each man went on his own accord and they did not seek advice from each other.” This Midrash teaches us that Nadav and Avihu did not actually go to offer the incense together, rather they each had the same idea and went alone to offer the incense in the Mishkan. They are criticized because they did not seek advice from their Rebbe, Moshe Rabbeinu, before undertaking this bold act, and also because they did not seek advice from each other. Rav Berel Soleveitchik zt”l asks that this Midrash is very difficult to understand; it is obvious why they should have consulted Moshe Rabbeinu because he would have surely advised them to not offer the incense, however why are they criticized for not consulting with each other? They both evidently believed in the correctness of their plan and so what benefit would have been gained from consulting each other - surely they would have merely confirmed that the plan was a good one?!
Rav Soleveitchik answers that we learn from here a fundamental principle in human nature; A person may want to commit a certain sin and yet he may simultaneously see the flaw in such an action when his friend is about to commit the very same sin. This is because each person is greatly influenced by his yetser hara which prevents him from making decisions with objectivity. Rather, the yetser hara clouds his reasoning and causes him to rationalize that it is acceptable to undertake certain forbidden actions. However, when this same person looks on his friend about to perform the very same sin he is able to take a far more objective attitude towards his friend’s actions. This is because with regard to others a person is not clouded by a desire for self-gratification and he can more accurately assess the validity of his friend’s plans. Accordingly, had Nadav consulted Avihu about his plan (or vice versa) then there would have been a good chance that Avihu would have seen the flaw in his brother’s reasoning despite the fact that he planned to do the very same act! That is why they are criticized for not consulting each other despite the fact that they both planned to do the same sin.
Rabbeinu Yonah brings out this principle from the teaching in Avos: “...Acquire for yourself a friend.” He writes that one of the benefits of having a friend is that he can help you in observing Mitzvos. “Even when a friend is no more righteous than him and sometimes he even acts improperly, nonetheless he does not want a friend to do the same [action], because he has no benefit from it.” He then brings as a proof to this idea the principle that “a person does not sin on behalf of someone else.” This means that a generally observant person usually sins because he is blinded by some kind of desire for pleasure, however with regard to someone else we presume that he is not blinded in the same way and therefore we do not suspect him of sinning on behalf of others. This idea is applied in a number of places throughout the Gemara. Rabbeinu Yonah thus teaches us the importance of acquiring at least one friend who can act as an objective onlooker towards our own actions, and that this friend need not necessarily be on a higher level than ourselves.
We learn from these ideas a very important life lesson; a person should not rely on his own assessments of his actions - it is impossible to be purely objective when making decisions because of one’s natural subjectivity that causes him to rationalize the validity of doing certain aveiros. Rather, he must realize the necessity of finding a friend who will be prepared to offer advice and even rebuke when necessary when he sees that his friend is blinded by his desires. May we all merit to acquire true friends who can help us find the true path of Avodas Hashem.
 Shemini, 10:1-2.
 Toras Kohanim, 1:32; this is a Midrashic work written specifically on the book of Vayikra.
 Quoted by Tallelei Oros, Parshas Shemini, p.165-6.
 Avos, 1:6.
 Rabbeinu Yonah, Avos, 1:6.
 Bava Metsia, 5b, Kiddushin, 63b, Shevuos, 42b, Arachin, 23a.