Moar C.S. Lewis! Prayer isn't an obligation in this case, and it serves not only God (presumably why we pray), but is also beneficial to the individual praying. Lewis recognizes both parties in the relationship and how important both God and human beings are in this dynamic.
LOVE THIS! LIVE THIS! When I stand before God at the end of my life, I would hope that I would not have a single bit of talent left, and could say, ‘I used everything you gave me.’ - Erma Bombeck 20 Funeral Quotes for A Loved One's Eulogy