This scholar was created by Zhu Dan Xi who lived AD1281-1358 in China. He thought that the Yin energy is more easily becomes deficient than the Yang energy. This must be the result of his personal clinical experiences. He used some interesting examples to explain this. The main causes that makes the defiency of Yin the sexual desire and nervousness. Sexual desire, sexual excess and mental nervouness creates the excess of unnecessary fire in the body and it dries up the Yin energy. He also uses some examples of nature, for example, the moon, which is the Yin energy, changes its shapes and sometimes becomes small. But the Sun, which is the Yang energy, never changes its shapes. That is why the Yin energy easily becomes deficient but Yang energy doesn't.
I can see many cases of Yin deficiency in this modern times. Perhaps it well fit with modern people's health problems. I think that each epoch has its own characteristics of health. Some epoch , the contagious diseases are important in the human body, and another epoch has the problems of deficiency....It is difficult to say only one problem. It depends epoch and persons.