To understand this chapter, we have to go back to the last verse in chapter 9, "And fire came out from before the Lord and consumed the burnt offering and the pieces of fat on the altar, and when all the people saw it, they shouted and fell on their faces."
The authorized fire that Aaron and his sons were commanded to use for the altar of incense was the fire from the altar of sacrifice that they were to keep burning day and night. This fire had been lit by God himself when He accepted the sacrifices made on it at the consecration of the tabernacle.
Aaron and his sons, like those who serve in ministry today, bear a greater responsibility because they are leading others by their example. If they treat the things of God as common, those who follow them will do the same. If they fail to give honor and reverence to the Lord, for He is Holy, how can they expect the congregation to honor God?
This authorized fire was based on sacrifice where the blood of the sacrifice was poured. The "pleasing aroma" was not just the concoction of spices that created the incense, it was the atonement of the sacrifice. "Without the shedding of blood, there is no remission of sin." (Hebrews 9:22) The incense represented intercessory prayer (Revelation 8:4), but without atonement, there is no intercession.
I confess that I sometimes approach God casually, as if Christ's sacrifice on my behalf means I can approach a Holy God like I would my BFF.
But I need to remember that Christ is my intercessor (Hebrews 7:25) and my Great High Priest (Hebrews 4:14-16). I can only directly approach the Holy and Righteous God the Father through the Son. Otherwise I, like Abihu and Nadab, would be offering unauthorized fire. I'd be trying to cut to the chase and worship God without going through the One who offered Himself as my sin offering.
I'm so thankful that we can approach Jesus with the intimacy of a friend and that He then carries all our burdens to the Father. His offering is perfect and acceptable to God the Father so that our prayers are heard.