Through our freewill, a divine gift which gives us autonomy and responsability for our actions, also through the law of cause and effect that shows us that every effect has a similar cause, and the multiplicity of existances which through reincarnation gives us the opportunity to continue our learning continuously, we can conclude that the majority of our present sufering has an origin in our attitudes from the past, in this or in previous incarnations, attitudes that we may or may not remember and consequentely my or may not understand.  

921. We can conceive that man will be happy upon the earth when the human race shall have

been transformed; but, meanwhile, is it possible for each man to ensure for himself a

moderate amount of happiness?

"Man is more often the artisan of his own unhappiness. If he obeyed the law of God, he would not only spare himself much sorrow, but would also procure for himself all the felicity that is compatible with the grossness of earthly existence." He who is perfectly sure that the future life is a reality regards his corporeal life as being merely a traveller's momentary halt in a wayside inn, and easily consoles himself for the passing annoyances of a journey which is bringing him to a new and happier position, that will be all the more satisfactory in proportion to the completeness of the preparations he has made for entering upon it. We are punished, even in the present life, for our infraction of the laws of corporeal existence, by the sufferings which are the result of that infraction and of our own excesses. If we trace what we call our earthly ills back to their origin, we shall find them to be, for the most part, the result of a first deviation from the straight road. This deviation caused us to enter upon a wrong path, and each subsequent step brought us more and more deeply into trouble.

(Source: The Spirit’s Book, Fourth Book, Chapter I)

171. What foundation is there for the doctrine of reincarnation?

"The justice of God, and revelation, for, as we have already remarked, an affectionate father always leaves a door of repentance open for his erring children. Does not reason itself tell you that it would be unjust to inflict an eternal privation of happiness on those who have not had the opportunity of improving themselves? Are not all men God's children? It is only among selfish human beings that injustice, implacable hatred, and irremissible punishments are to be found." All spirits tend towards perfection, and are furnished by God with the means of advancement through the trials of corporeal life; but the divine justice compels them to accomplish. in new existences, that which they have not been able to do, or to complete, in a previous trial.

It would not be consistent with the justice or with the goodness of God to sentence to eternal suffering those who may have encountered obstacles to their improvement independent of their will, and resulting from the very nature of the conditions in which they found themselves placed. If the fate of mankind were irrevocably fixed after death, God would not have weighed the actions of all in the same scales, and would not have treated them with impartiality. The doctrine of reincarnation-that Is to say, the doctrine which proclaims that men have many successive existence-is the only one which answers to the idea we form to ourselves of the justice of God in regard to those who are placed, by circumstances over which they have no control, in conditions unfavorable to their moral advancement ; the only one which can explain the future, and furnish us with a sound basis for our hopes, because it offers us the means of redeeming our errors through new trials. This doctrine is Indicated by the teachings of reason, as well as by those of our spirit-instructors. He who is conscious of his own inferiority derives a consoling hope from the doctrine of reincarnation. If he believes in the justice of God, he cannot hope to be placed, at once and for all eternity, on a level with those who have made a better use of life than he has done but the knowledge that this inferiority will not exclude him for ever from the supreme felicity, and that he will be able to conquer this felicity through new efforts, revives his courage and sustains his energy. Who does not regret, at the end of his career, that the experience he has acquired should have come too late to allow of his turning it to useful account? This tardily acquired experience will not be lost for him; he will profit by it in a new' corporeal life.

(Source: The Spirit’s Book, Second Book, Chapter IV)