It is normal for an individual to experience irritability and anger. Some individuals, however, experience excessive and/or uncontrollable anger which often leads to them becoming verbally or physically aggressive with others.
Therapy can help individuals problem-solve around why their behavior is maladaptive, identify their triggers, learn positive coping skills (including relaxation exercises and thought restructuring techniques) to calm themselves down, and learn ways of expressing their thoughts/feelings in an assertive instead of aggressive manner.