This is an essential quality for talented diplomats. By becoming a diplomat, you often have to present your views on a certain issue on behalf of your country, warn the opponent or enlist your country's agreement. In other cases, the diplomat must use words to hide national secrets, the strategies to be kept secret of his country, and still make the enemy believe.
The diplomat must always be the most polite and courteous. No matter how frustrating they are, they always try to stay calm, hiding their national secrets and thoughts.
If the person you are talking to has gestures or statements that make you want to be mad, what will you do? Give him a slap? Or a cellar?