There is a real, hidden reason behind many arguments. They're not really arguing over a chocolate bar or a toy: actually what they're fighting over is their parents' affections.
As soon as a baby brother or sister arrives and the mother-child relationship, the dyade according to psychoanalysts, is complicated by the arrival of a new child, the older child wants that old unique love that he had from his parents (and especially his mother) back. This is why they seek to eliminate the young pretender to their parents' love. It's a basic, inevitable form of jealousy that can even be essential to growth, but it emerges when they argue and can even turn violent.