In this life, happiness or unhappiness is results of karma and karma is a truth

At this time, we have obtained a human body that has a human existence in this world. When we have the pleasant and unpleasant experiences of human beings we express different states of happiness and unhappiness. We are like children who wander through happiness and unhappiness, laughing and smiling one moment and crying the next.
What is the cause of our happiness and unhappiness that change like the clouds in the sky?