Understanding the Qualities of Yin and Yang

Why does Yin turn into Yang, and Yang turn into Yin?

Yang is forceful and powerful but inevitably exhausts itself. Yin is passive and accepting and through accepting gradually builds itself up.
(When Yang exhausts itself it will become Yin, when Yin builds itself up enough it will become Yang)

Yang is a "strong" power, but this means that it is more unstable. It is easier for it to change into Yin.

Yin is a "weak" power, but this means that it is very stable, it takes a lot for it to change to Yang.

This is also why it is easier in our reality for something strong to become weak than for something weak to become strong (the law of entropy).

Hence, Yin only becomes Yang when it is so totally 'full' of Yin that it cannot help but actually transform. 

And when Yang becomes 'too Yang', it will sooner or later break down into Yin.