What did C. S. Lewis base his idea of sehnsucht/joy on?

C. S. Lewis' idea of *sehnsucht* is that we sense that there is some ineffable object to our desires and that this proves that there is something beyond this world and life (for it makes the enjoyment of those goods pale in comparison). He used this idea, in one instance, as proof of God. What evidence, whether from the Bible or philosophy or his own experience, did he base this idea on?