First of all, I want to say that I believe on God very much. So my explanation is also related to God. You know that God rewards if you do good. He also punishes if you do bad. There is a popular saying in the world, ''You will reap what you sow.'' But how can God keep records of all mistakes of all people of the universe? It is very impractical. So God made a rule. I call this rule as ''The Spiritual Law of Nature''. Due to ''Spiritual Law of Nature'' people are suffering in their life. Let me explain it in detail.
According to 'The Spiritual Law of Nature'', humans get same returns as they act out. It means if you do good today, you will see good day tomorrow. And if you do bad today, you will see bad day tomorrow. So if you are getting happiness today, it is the reward of your good work yesterday. Similarly if you are suffering today it is the returns of the bad work you did yesterday..
Discovered by +Don Prince