Why did the Buddha require parental permission for a son to be ordained as a monk?

The requirement that a son must obtain parental permission before being ordained as a monk was established by the Buddha following the ordination of his own son, Rahula.1

When Rahula was sent by his mother, Yashodhara, to ask the Buddha for his patrimony, the Buddha responded by having the boy ordained. This action caused great distress to the Buddha’s father, King Suddhodana, who had already suffered immense pain when the Buddha himself had renounced the world. To prevent such suffering for other families, the King asked the Buddha to ensure that, in the future, a son would only be ordained with the consent of his parents. The Buddha subsequently incorporated this request into the rules of the monastic order.1