Anger acts like a poison on selfishness, and we find that in time it gradually transforms the powers of the soul and makes it capable of love. […]
Anger works inwardly and can be regarded as a regulator for unjustified outbursts of selfishness. Anger must be there or it could not be fought against. In overcoming anger the soul continually improves itself. If a man insists on getting something done that he considers right and loses his temper over it, his anger will dampen the egoistic forces in his soul; it reduces their effective power. Just because anger is overcome and a man frees himself from it and rises above it, his selflessness will be enhanced and the selflessness of his Ego continually strengthened.
Source: Rudolf Steiner – GA 58 – Metamorphoses of the Soul / Paths of Experience – Volume One: Lecture 3: The Mission of Truth – Berlin, 22nd October 1909
Translated by C. Davy and C. von Arnim