The theory of karma simply says that whatsoever is happening to you is your doing. Nothing happens to you from outside of you. You create whatever happens to you. Most of the time you are not aware and you are not conscious. So you are not able to see yourself doing it.
If you are sad, watch and meditate on your misery. Observe how you created the sadness. See how the root of your misery lies within you. The mind constantly tells you everybody else is responsible for your misery, spouse, friend, boss, or the man down the street. It always goes on throwing responsibility on somebody else.
Be very clear, nobody except you is responsible for what you feel, what you do.
Nobody can make you suffer without your silent permission. Once you understand that whatever happens is from deep inside you, you will suddenly see things changing. Take full responsibility for yourself and see how you become integrated in whatever you do.
When you stop blaming others, when you understand that you are responsible for everything, you will become more and more aware of every action, every thought of yours.
That awareness is enough. You will act intelligently. If you want to be sad, you will be sad. Nothing wrong with that, just be total in that sadness. If you feel there is no need to be sad and you want to be happy, be happy. That’s all.
Watch how you create your misery. How did you build it inside? Watch how you are continuously creating your moods. Don’t fool yourself by throwing on others the responsibility of what you create. Then you will never change. You will remain miserable because you are helpless. If you think others are creating everything, what can you do? What is your role? Then nothing is in your hands. You can only wait for others to change. But the change can never happen because others are not creating your feelings and emotions in the first place.
source: Living Enlightenment