“Elazar the Kohen said…‘This is the Torah decree which God commanded Moses.”
~ Numbers 31:21

He’s called ‘Moses our teacher’ because he taught us all of the Torah. Of the hundreds of laws, he said all but two.

Why did Elazar teach this specific one?

Because Moses forgot it.

He was in a temper about something and the law simply escaped him, even though he was right. Like inebriation, anger numbs the senses, reduces alertness and causes temporary amnesia.

When driving, we avoid both drunkenness and rage. Should the rest of life be any different?