Because anger is a map. Anger shows us what our boundaries are. It tells us that old life is dying. It tells us we are being reborn, and birthing hurts. The hurt makes us angry.
Anger is the firestorm that signals the death of our old life.
Anger is the fuel that propels us into our new one. Anger is a tool, not a master. Anger is our friend. Not a gentle friend. But a very, very loyal friend.