Mere Christianity, in The Complete C. S. Lewis Signature Classics

p. 34

“God created beings which had free will.  That means creatures who can go either wrong or right.  Some people think they can imagine a creature which was free but had no possibility of going wrong.  I cannot.  If a thing is free to be good it is also free to be bad.  And free will is what has made evil possible.  Why, then, did God give us free will?  Because free will, though it makes evil possible, is also the only thing that makes possible any love or goodness or joy worth having.  A world of automata—of creatures that worked like machines—would be hardly worth creating.  The happiness which God designs for his higher creatures is the happiness of being freely, voluntarily united to him and to each other in an ecstasy of love and delight compared with which the most rapturous love between a man and a woman on this earth is mere milk and water.  And for that they must be free.”