The Danger of Anger
Moshe was angry with the commanders of the army... (Num. 31:14)
The laws of koshering utensils were transmitted by Elazar and not by Moshe because Moshe had become angry and forgot the laws. The difficulty with this is that Moshe was certainly justified in getting angry. God told the Jews to wage a war against Midian, who were responsible for causing the Jews to commit acts of immorality and idolatry. The troops killed all the males but spared the Midianite women - the ones who were primarily responsible for inciting those sins. Moshe thus became angry at the officers who allowed this. Why then was he punished?
Imagine that you try to open your car door and the key won't turn. You begin to get frustrated and try to force the key to turn - jamming the key inside and causing considerable damage. If you had taken a step back and thought rationally, you would have realized that if the key is having difficulty turning then you must have stuck in the wrong key! When someone gets angry he turns off his intellect and acts on his instincts. He doesn't think rationally any more. Anger and intellect are a contradiction to each other.
The fact that the laws were not transmitted through Moshe was not a punishment. Rather, when Moshe became angry his instincts took over and his wisdom was removed from him. He could not remember the laws and so they had to be transmitted through Elazar.
Anger can destroy years of investment in a relationship in a matter of minutes. It destroys one's health, eats away at happiness, and causes a person to lose his ability to think clearly. The trick is to hold yourself back at that initial instinctive reaction to retaliate; bite your tongue for just a few moments and then the anger will subside, enabling you to act rationally.
1. R' Chaim Shmulevitz.