Anger is what you feel when something is said or done that directly opposes something you strongly believe in. Anger is nature's way of alerting you that you are incompatible with a person or situation. The extent to which you feel angry about a person or situation is the degree to which you are incompatible.
This explains the observation that, the more religious, nationalistic, or dogmatic a person is, the more angry that person tends to be. The more strongly you believe in something, the more often something is said or done that directly opposes your beliefs. So the more open-minded a person tends to be, the less angry he or she generally is. We all have our own set of beliefs and we all hold some things sacred, but the difference is that the open-minded person shows respect and tolerance; thus allowing room for others to live according to their own beliefs.
This way of understanding anger also explains why it is generally a bad idea to preach, to judge, or even to offer advice to others. The way to avoid anger in your life is to express your own convictions as your own, and allow others the space to do the same. Through tolerance and open-mindedness, even two people who hold conflicting beliefs or opinions can achieve compatibility. They can experience the beauty of life without anger.