The second Usui Reiki principle for happiness is, "Just for today, do not anger." Usui believed that anger was often a destructive emotion because it's usually expressed in an explosive manner which simultaneously hurts ourselves and others. Usui believed anger resulted from having expectations about what we think should or should not happen. Choosing to express anger is a conscious choice. Even though you can't choose how you feel, you can choose how you behave. When you feel angry it’s important to be honest with yourself and to express it in a safe, non-destructive way.
Typically there’s a lot of shame associated with feeling angry, but there's no need to feel ashamed because anger is usually a sign that something needs to be addressed. Was a boundary crossed? Did you perceive something differently than the reality of the situation? Were you disappointed by something that you expected, but didn't happen? Learning how to express anger in healthy ways will support your emotional health by creating healthier relationships, including the one with yourself.