Anger blinds the mind. Anger renders clear, rational thinking impossible, and blinds the mind. It takes a long time and great effort to recover to a peaceful, cheerful and clear mind afterwards. Anger damages relationships with others. Anger makes one lose control and perhaps commit actions one could later regret.
Emotional damage is very hard to forget in a short time.
And whatever harm one does to others will eventually bounce back to oneself. Through Qigong practice, one's thought, emotions, consciousness, may become highly developed. To be angry is to risk the danger of intentionally or unintentionally using one's developed power to harm others. To be angry means something or someone falls below your expectation. I think a Qigong practitioner is not supposed to expect things from others, or impose certain standards on others.