Proverbs 16:32

He who is slow to anger is better and more honorable than the mighty (soldier). And he who rules and controls his own spirit, than he who captures a city.

It is outside forces of heat that cause water to boil. People who control their anger and spirit do not let external forces rule or influence their decisions. Thinking and making sound decisions is more straightforward when not being ruled by anger. Anger can lead to decisions made in haste, and a spirit that is controlled will not react in haste.