Our life are just like a glass of water,
If you don't fill it with water, it won't be full
If you fill it to much it will be overflowed
So, we had to adjust it.

Just like our life of happiness and sadness
If we feel too happy then, we couldn't help others when they are sad, because you, yourself had never once feel it
But, if we are too sad then, we couldn't overcome it, not without happiness

Therefore, everything need an adjustment to be balanced.