The ancient Chinese YIN-YANG scholars thought the universe is divided into three parts. The first part is Heaven, the second part is Earth and the third one is for all living species. Because humans are in charge of all the living species, therefore the third one is named Human.
We can find Metal, Water, Wood, Fire and Earth everywhere on the ground. The Chinese scholars named them the Five Elements. Human beings live between Heaven and Earth just like Five Elements. Therefore humans must have a relationship with Five Elements or be like them. As there are affinity and enmity relationships among the Five Elements, there must be similar relationships among humans.
For example, trees have their growth cycle in between Heaven and Earth. Humans must have life cycles similar to those of trees. The seasonal changes affect the growth of trees. Trees grow faster in the spring and slower in the fall. The environmental changes also affect the life of trees. Without adequate sunshine, trees grow too slowly. Too much heat, trees will be dried out. Without water, trees cannot grow. Too much water, trees will be uprooted and afloat. Insufficient earth, tree will not grow tall....
The human life cycle is also affected by the same seasonal and environmental changes. The seasonal changes come from the Sun and Moon which are the clock for the calendars. The Chinese calendar is designed from Sun and Moon plus the Stem-Branch (concept from trees) cycle. Chinese YIN-YANG scholars have for thousands of years applied the Five Elements on the Chinese Stem-Branch calendar. This way humans get their Five Element weights using their birth date and time.
When the seasons and environment change, the Five Elements have certain responses. Humans respond in a similar way. The YIN-YANG scholars made predictions on the human life cycle, from birth to death, by using this natural phenomenon.