Imagine someone buying a new car. One more thing - your instruction manual. I instinctively feel what the tire pressures should be, and I have a sixth sense when the car needs a major service. I know intuitively what octane fuel the car needs. Few people when faced with operating something as precise and unforgiving as a car would leave these sorts of decisions to instinct and feeling.
Life is no less demanding nor complex than a car.
And yet many people are happy to coast along, assuming that they are not putting water in their spiritual gas tank. The purpose of life is to become close to the Creator of the world, and only the Creator of the world knows how the world can be utilized to become close to Him. We live in an era where people are more interested in feeling spiritual than being spiritual . That is what the Torah is warning us against in the story of Nadav and Avihu. If it was a strange fire, then by definition it was not commanded by Hashem.