The quick and unfailing responsiveness of souls is expressed in the law that hate begets hate, lust begets lust, and love begets love. This law operates not only during a single lifetime but across several lives. An individual feels impelled to hate or fear an enemy from past lives, although the present life may not have provided him with any apparent reason for this attitude. In the same way, without any apparent reason from the present life, he is impelled to love and help a friend from past lives. In most cases the person may not be aware of the reason for his unaccountable attitude, but that does not mean there is no reason for it. Many things that seem inexplicable on the surface become intelligible when considered in the light of karmic links brought forward from past lives.
The law of karma is law exhibiting itself through continuously changing mutual adjustments, which must go on when there are individual souls who seek self-expression in a common world. It is an outcome of the responsiveness of ego-minds. The rhythm in which two souls start their relationship tends to perpetuate itself unless the souls, through fresh intelligent karma, change the rhythm and raise it to a higher quality.
-Discourses 7th Ed., p330