When the second child is born, the first child experiences a devastating dethronement. S/he can no longer imagine that s/he is the focus of the parents' world.
My mother experienced the birth of a sibling when she was five, and concluded that her parents no longer wanted her. She went down the street to a nice-looking house, rang the doorbell, and asked the maid if the owners would be interested in adopting her. Her mother countered by telling her that she was needed to look after the baby -- a burden that haunted her life. Every time my grandmother staged one of her deathbed scenes, it included a promise to look after the baby brother. That weighty sense of responsibility, coupled with the repressed desire that something horrible would happen to the little monster, created a conflict which was still alive and well when the "baby" was seventy-five years old.
Love never seems to be enough for the dethroned child. He craves uninterrupted, unqualified admiration -- what he got when he was that miraculous first child. To me (a non-dethroned only child), being the "good child" means being the child who contributes the most; to the dethroned child, it means being the child who is adored the most.