The law of cause and effect, action and reaction, applies to everything in life and forms the basis of our Karma.
The results of all the causes we have made in this, and previous lives, is precisely why we are where we are today.
The more good causes we create, generally speaking, the more good effects we see, and the happier we are with our lives.
Dora, in the book The Buddha, Geoff and Me, explains that Karma is a bit like letters we write to ourselves. Many of those letters were written so long ago, that we have forgotten all about them.
The nice letters are lovely surprises when they arrive. Nasty ones come as a bit of a shock, they may annoy us and we might even write another nasty one in response. Of course, in time, those responses get delivered too, so the cycle can repeat itself, time after time.
That is, of course, until you understand the way the process works. Once you realise that you create the causes, you can create causes for nice, or good effects, rather than going round and round forever.
I cannot imagine a more important lesson to learn, to know that your Karma, and hence your future, is determined by you? It is the most empowering feeling, to take control of your life and to have your destiny in your own hands.