Leviticus 9 ended with fire coming out from the presence of the LORD and consuming the sacrifices on the altar. It was an auspicious sign. The people shouted in ecstatic worship and prostrated themselves before God. Leviticus 10 opens with fire coming out from the presence of the LORD and consuming the high priest Aaron's sons, Nadab and Abihu. The juxtaposition of these two stories is supposed to teach us a lesson about God. In both stories the fire came out from the presence of the LORD. In both stories the fire represents God's glory. It is the same fire. It is the same God.
Then Moses said to Aaron and to his sons Eleazar and Ithamar, "Do not uncover your heads nor tear your clothes, so that you will not die and that He will not become wrathful against all the congregation. But your kinsmen, the whole house of Israel, shall bewail the burning which the LORD has brought about." (Leviticus 10:6)
On his first day of service in the priesthood, Aaron lost his two oldest sons in a tragic accident. Nadab and Abihu inadvertently transgressed God's sanctity, and they died in a supernatural conflagration.
Moses did not allow Aaron and his sons to mourn the deaths of Nadab and Abihu. As priests serving in the Tabernacle on behalf of all Israel, they had to set aside their personal feelings for the duration of their shift of duty.
Mourning was not allowed in the Tabernacle because the Tabernacle represented God's dwelling place, where one day He "will wipe away every tear from their eyes; and there will no longer be any death; there will no longer be any mourning, or crying, or pain" (Revelation 21:4). Aaron and his surviving sons had to stifle their personal pain and continue the service. It is difficult to imagine.
Therapists encourage people to let their feelings out and to avoid bottling up emotions. This may be a good policy while in the therapist's office, but it is not a good way to conduct life. Of course we should be honest about our feelings, but it is not always appropriate in every situation. A person who wears his heart on his sleeve is ultimately demonstrating selfishness. Because he feels pain, he wants others to feel it too. An angry person disregards the feelings of others as he gives vent to his emotions. A person in love forces his attentions on the unwilling target of his affection. It's not a proper way to behave.
Aaron and his remaining sons held their grief in abeyance while they finished the day's sacrificial duties. Nevertheless, Aaron did not feel it was appropriate to eat the meat of the sin offering while his heart was so heavy, so he had the goat burned. Moses rebuked him, but Aaron explained, "When things like these happened to me, if I had eaten a sin offering today, would it have been good in the sight of the LORD?" (Leviticus 10:19). The offerings were to be eaten with rejoicing. Aaron was willing to do his duty in the Tabernacle, but he was not willing to fake a spirit of religious joy before the LORD.