Siblings fight for a number of reasons. They fight because they want a parent's attention, and the parent has only so much time, attention and patience to give. They must love him more than they love me. Children need not weeks or months but years to learn some of the socially approved ways to behave in relationships. Lessons about jealousy, competition, sharing and kindness are difficult to learn, and, indeed, some adults still haven't learned them.
Adolescents fight for the same reasons younger children fight.
But adolescents are bigger, louder and better equipped physically and intellectually to hurt and be hurt by words and actions. What parents may forget is that adolescents are under pressure from many different directions. Physical and emotional changes and changes in thinking cause pressures, as do changing relationships with parents and friends. Teens may be concerned about real or imagined problems between their parents. They feel pressure about the future as adults and about learning to be an adult.