The Vedic literature explains that human activity, when devoid of service to the Lord, is governed by a subtle law known as the law of karma. This is the familiar law of action and reaction as it pertains to what we do in this world and the enjoyment or suffering we experience as a result. If I cause pain to another living being, then as surely as the wheel of life turns, I will be forced to suffer similar pain. And if I bring happiness to another, a like pleasure awaits me. At every second, with every breath, our activities in this material world cause enjoyment and suffering. To facilitate these endless actions and reactions, there has to be more than just one life. There has to be reincarnation.
[From "The Laws of Nature: An Infallible Justice"]
