Yin-Yang is an ancient Chinese symbol, which tells how seemingly opposite forces are interconnected and interdependent. The two opposite colors stand for diametric things in life, such as day-night, light-dark, good-bad, happiness-sadness, success-failure, pleasure-suffering so on and so forth. Basically, one can relate everything in this world to the yin-yang. Everything has a 50-50 chance of occurring. Nothing is permanent in life. So if you are sad today, be sure that sooner than later happiness is bound to come your way. On the contrary, if you are at the top of the world today, at the zenith, don't be devastated when you are nearing nadir. So, one very strong message that the yin-yang sends across is that be nice to people on your way up, as you will meet them on your way down.
One more thing which you might notice is that there is a tiny black circle in the white portion and vice-versa. This means that the
two states are not completely independent of each other. Thus, there is no state which is 100% true in life. In your "happy" state you might have bouts of sadness, and obviously you can't be cent percent "sad", as your near and dear ones are there to cheer you up! Just like there is a bit of darkness in the day (signs of pollution) and a bit of day in the dark, when the starts shine bright.; or a bit of sun in the winter and bouts of rainfall in summer.....
I feel that I can relate almost everything in life to the all-encompassing yin-yang. Try it you can!