Here's his two part explanation:
1. "On the one side it is intended to account for the fact that people are endowed with different mental capacities and their lives proceed along idiosyncratic lines that cannot be satisfactorily explained in terms of environmental or other external factors."
2. "On the other side, far from relieving a person from all responsibility, thee teaching of reincarnation constitutes a challenge to actively determine his or her future lot. Nothing would be more wrong and destructive than to regard the doctrine of repeated births as a convenient excuse for a fatalistic attitude. Rather it should be seen as an urging us to accept our individual 'starting 'point, however disadvantageous it may seem, as the direct outcome of our previous mental activities, and to make the best of our life within the given parameters."