Only three years after his birth does a child leave the arms of his father and mother, so the rule that three years of mourning be observed after the death of a parent is a way of returning this parental kindness.
期年之喪通乎諸侯，三年之喪通乎天子，禮之經也。
Eric Henry
That a year of mourning should follow the death of a regional lord and three years after the death of a Son of Heaven are basic tenets of ritual.