This is one of the five precepts of Reiki.
Anger is a natural instinct found even in the animals. It is a reaction of the conscious mind to what we do not like and is generally more than just the disliking. One can also be angry when wronged in any way. Thus becoming angry is not uncommon in daily life but frequent anger can easily make a person irrational, which leads not only to incorrect decisions but also to strained relationships.
Anger can be impulsive and short lived or prolonged and persistent. Both ways it tends to eat up the wisdom. Impulsive and sudden anger usually results in quick but irrational decisions and reactions. Impulsive reactions might even be indecent and violent and provoke similar reaction from the other side worsening the situation.
During the spell of anger, everyone does not become violent but is very likely to become emotional, take an irrational decision, act wrongly, curse or use violent language. It is important to understand that the angry person would first tax his own nerves before he transfers his feelings to the other person. Thus the first victim of the anger is the angry person himself. It is therefore considered a negative emotion that attracts negative vibrations of energy. Continued or persistent anger accumulates negative energy that ultimately translates itself to physical shape giving birth to various diseases.
Islam taught us over 1400 years back not to be angry and learn to forgive. It would be wise to understand the wisdom behind this precept and avoid being angry. It is extremely difficult to eliminate anger all together but can be kept harnessed. Anger is like a wild horse; not easy to tame, but it can be done by being more tolerant with some conscious effort. It is certainly not impossible. Use Reiki to naturally calm down for it makes the person more realistic and accommodative. If someone wrongs you, try to forgive him or leave it to God for He is always just and will rightly decide for the guilty. So better avoid being angry, learn forgiveness or else you will only cause suffering to yourself, nothing else.