Are you more a rule follower or one who likes to act as pleases you in the moment? We have both types of people in our tradition. The former is Moses, who as G-d’s emissary laid out the rules by which Israel needed to live. The latter are Aaron’s sons Nadav and Avihu who spontaneously offered “a strange fire which G-d did not command of them.”
What was this foreign fire? Some say it was idolatrous, as אש זרה is very similar to עבודה זרה, the Hebrew term for idolatry. Others say Nadav and Avihu were drunk. Yet others say the offense was merely that G-d did not command them to offer this fire.
The irony is that Aaron in our tradition was reluctant to serve as a Kohen. The text reads ויאמר משה אל אהרון קרב אל המזבח “Moses told Aaron ‘approach the altar.’” Why did Moses have to tell Aaron rather than Aaron approaching on his own? Rashi comments that Aaron was embarrassed over having partook in the golden calf and needed a push from Moses to know that G-d wanted him to serve as a Kohen. He needed affirmation that he was chosen for this holy work. Ramban adds that Aaron had in mind וחטאתי נגדי תמיד, “my sin will be before me always” and that he could not atone for his past, yet Moses told him that his work in the Tabernacle would serve as his atonement, כפרה. This indicates that Aaron’s humility is what enabled him to become Kohen Gadol.
Unfortunately Aaron’s son react differently, wanting to feel self-important through offering a sacrifice of their own. They wanted their offering to be accepted and in the zeal of the moment they offered it directly before G-d. There are traditions that they even saw G-d’s face and we know from Exodus לא יראני האדם וחי, “no man shall see me and live.” Rather than acting in accordance with the rules of Leviticus, the priestly manual, or תורת כהנים, Nadav and Avihu followed their hearts’ desire-and paid for it dearly with their lives.
What does this teach us? Must we never be spontaneous and always be rule followers? I would not go that far; rather it demonstrates the importance of taking care when engaged in holy matters. Aaron’s caution and reluctance is what merited him becoming Kohen Gadol in the first place. His sons’ brazenness in acting on their own is what led to their demise. In matters regarding the Divine one must take extreme care.
Shouldn’t we celebrate Nadav and Avihu’s zeal to give their own offering to the Almighty? Not in this case: we need to recognize that there’s a time and place for everything including tzimtzum: recognizing you’re not the right person to do something and restricting yourself. Another story about this is from the time of King David when the oxen pulling the Ark of the Covenant stumbled, and a Levite named Uzzah took hold of the Ark. G-d was displeased and struck him down. Why would G-d do such a thing? Because the only people supposed to touch the Ark were a subset of Levites called the Kohathites, and Uzzah was not one of them. He was not qualified for such holy work. While he had good intention, he needed to think before he acted if this was what he was called to do.
There’s a great story of four rabbis who went into paradise (פרדס) to seek out G-d. Only one of them came out with שלום, which I translate as “peace of mind.” Ben Azzai died, Ben Zoma went crazy, Elisha ben Abuya (known as “the other one”) became an apostate. Only Rabbi Akiva went in and came out with peace of mind. One must be prepared for such an intense spiritual encounter. It’s like not being able to study Lurianic Kabbalah until one is 40 with children, with enough life experience and other learning to be prepared for what s/he will encounter.
When we are in a situation where we think it is time to break rules, let us first examine why those rules are in place and whether or not this particular situation calls for spontaneity. In so doing, may we avoid the pitfalls of Nadav and Avihu and have the humility of Aaron, being certain before we act that we are precisely the person who is called to do the given task.
 Leviticus 10:1
 Leviticus 9:7
 Rashi on Leviticus 9:7 ויאמר משה אל אהרון קרב אל המזבח ד”ה
 Rashbam on Leviticus 9:7 ויאמר משה אל אהרון קרב אל המזבח ד”ה
 Leviticus 33:20
 2 Samuel 6:1-7
 Babylonian Talmud Hagigah 14b