The adolescent brain
The brain makes sure that an adolescent becomes sexually mature. And adolescent behaviour too is caused by these hormones.
In puberty the pituitary gland below the brain stimulates the production of our sex hormones. These hormones affect the adolescent brain, producing conspicuous and often very annoying changes in behaviour. The evolutionary benefit of puberty is clear: children are being prepared for reproduction.
Adolescent behaviour, with the child distancing itself from the family, reduces the chance of reproduction occurring within the immediate family circle and thus lowers the risk of congenital deformities.