Minutes later, both Sally and Max were complaining they were bored and begged Mom to let them play together. Mom, they fight constantly when they're together, but they can't stand to be apart. Later that night, watching her now peaceful warriors cuddled up next to Dad listening to a bedtime story, Mom thought, it is the best of times. But she also felt a pang of sorrow, because she knew tomorrow would bring another day of conflict between her children another day of sibling rivalry. It's a question parents have been asking forever.
Siblings fight for lots of reasons.
A younger child may fight with his sister to prove he won't be bossed around the older one to prove she is the boss. However, the biggest reason for sibling rivalry is the need for exclusive parental love, approval, and attention. The intense desire each child feels to be first and best often triggers jealousy between siblings because each perceives that their parents love the other best. Imagine how threatened a child must feel when a new sibling arrives. His secure position in the family has been undermined, thus unleashing the hostility that leads to the struggle to be number one in his parents' eyes.