Why did the ancients first pay attention to the birth when calculating divination?

When the ancients divined, they first paid attention to the birth, because in ancient folk beliefs, it was generally believed that fate was decided by heaven, that is, everyone's fate was arranged by heaven at birth. And his date of birth, even the place, contains the information of fate. Therefore, the ancients believed that the fortune of a person's life (such as wealth, poverty, longevity, education, occupation, emotional marriage, career fortune, status of six relatives, etc.). ) can be calculated by birth, which is the legendary eight-character life number.