Shadowlands - Why do we pray?

Author C. S. Lewis' wife, Joy Gresham, is dying of cancer, prompting him to pray for divine intervention. A priest, Harry Harrington, tries to offer words of encouragement.

Harry: Christopher can scoff, Jack, but I know how hard you've been praying; and now God is answering your prayers.
C. S. Lewis: That's not why I pray, Harry. I pray because I can't help myself. I pray because I'm helpless. I pray because the need flows out of me all the time, waking and sleeping. It doesn't change God, it changes me.