Karma - one simplified explanation of this belief is that we create our own fate. We are the recipient of the effects of our own actions. I like the phrase, "What comes around, goes around." It sounds simple, but is in reality quite complex.
There seems to be a reaction to every action. As masters of our own fate, we are responsible for how we respond to life's challenges. That response may determine how happy or unhappy our life was, is or may become. Free will plays an important role.