Yin/Yang theory has been in existence for thousands of years, but most of us in the west are unaware of how it is part of everyday life. It is all about balance.
We all know the symbol well, but it is a static picture. Nothing is pure yin or pure yang and these two opposites are constantly changing and merging together, just as our lives are constantly changing and evolving.
When our lives are well balanced, we are happy. It only takes one element of our life (eg. work, relationships, etc) to be out of balance to cause stress which then affects all of our life including the good bits.
Stress or being unhappy is possibly the leading cause of disease which may simply be a stiff neck or headache, but may also lead to much more serious conditions.
Prevention is better than cure. Identify the cause of your stress or unhappiness and do what ever you can to fix it.