The only cause of happiness is love and the only cause of suffering is self-grasping. If you just understand this, you have understood the workings of karma, cause and effect, perfectly.
Loving kindness decreases our self-grasping and the mind becomes naturally expansive.
If you develop it in your mind-stream, you are the richest person. If you have developed it in your heart, it gives you a lot of happiness. This is how I survived in prison.
Loving kindness is the most precious thing.
Garchen Rinpoche on Karma