Every person is part of a grand chain of being. The success of their forebears, their ancestors, is a long linked system of relationships in which parents choose to grant their gifts, not just of their life, but of their lineage to an undiscovered human of their lineage. There is no more generous act than this. In receiving this great gift there can never truly be a means to pay back such a whole act. The only way to ever repay this gift is to make your own generous offering into the world in the same manner that you received, only in doing this can you be worthy of the gift unto yourself. To fail to make this offering is to fail that chain of generous lineage and to have failed to meet this debt. There is no worse failing than to have failed to join the great chain of being and to have left debts unpaid.
After all, what could be greater than to have lived and known. To have come from nothing and become something and to join the chain, the lineage.
To those selfish souls who choose to deny this debt or as a further reward for those who join the chain we can look to what brings humans the most joy. It is the mixing with those similar to them, sharing their experiences. From a mutual understanding does the sharing gain value. Sharing something with a complete unknown devoid of anything shared feels not like sharing at all but throwing something to the void, only slightly different than shouting at the stars or speaking with an ignorant animal. Who better to share with than those that understand you and are a part of you.
In children not only do you share so much but you also have the chance to remedy the wrongs in your own life by proxy or to grant the wisdom of your own life and bring about a better world by making a better person. In this we find further purpose, now not only do we repay great debts but we have a way of bringing ourselves into the future which otherwise our own mortality would prevent. What greater glory could there be than placing the bricks of the future for all to appreciate. This forms our legacy, part extension of ourselves that is brought into the future. We can also understand that in children which we are responsible for introducing to the world and equipping them for its navigation. Despite all the trials we might face in our lives, properly equipping children for the world offers the greatest chance for improvement of the world and the greatest chance for us to create the world we would like to see. If we instead choose to betray our legacy, not equip our children for the world then the opposite can be true. Our children will reject us if we are found wanting and a large part of what we can bring into the world will be undone, as part of such rejection, our children will refute what we are and weaken not only our legacy but our lineage.