In an attempt to explain why babes born during different months (in the case of the western horoscope), or during different years (in the case of the Chinese horoscope) have different personalities, humans have looked to the stars. The answer, however, is closer at hand, and is not anymore mysterious than the different personality shadings that siblings develop based on their birth order. The western horoscope describes spring babes as enthusiastic, summer babes as languid, fall babes as guarded, and winter babes as depressive - not unlike the seasons! Babes are highly sensitive to the moods of their caretakers, and adopt more than the mannerisms of those they take to be their role models.
If this explains seasonal horoscopes, what on Earth would affect the twelve year cycles the Chinese have recorded? There are many such cycles, noted even in the western world. New marriages must get past the seven year itch, and youngsters reach the age of reason at the age of seven, and the major phases through adulthood have even been noted to fall into such seven year cycles. Is this a type of biorhythm, a societal rhythm? It is indeed, as societies have a type of life, reflecting as they do the many lives of their members - somber years of marketing retraction followed by adventurous rebellion, years of caution followed by playful extravagance. In the west, such cycles are termed economic boom times or depression, conservative repression or high-flying extravagance such as the roaring twenties. The Chinese cycles were developed when the factors influencing societies moods were limited, and thus had more influence, where the more free wheeling west has more factors influencing its cycles.
Horoscopes thus are all very down to Earth.