What is the theme in A Grief Observed by C. S. Lewis?

Lewis spends a good deal of time wrestling with the contrast between his expectations and reality, and voices it most passionately as he expresses his seeming disillusionment with God. Thinking that God would be his most attentive sympathizer, Lewis finds himself kicking and screaming in his prayers for some comfort, and the doors of heaven seeming to be locked tight against him. For a man who so passionately sought honest and soul-refining communion with God as Lewis did, and who had set out to use his experience with grief as a way of teaching people who would follow him how to be in the state healthily, abandonment by God was a world-shattering thing to imagine.