From C.S. Lewis' "The Case for Christianity":
“God created things which had free will. That means creatures which can go wrong or right. Some people think they can imagine a creature which was free but had no possibility of going wrong, but I can't. If a thing is free to be good it's also free to be bad. And free will is what has made evil possible.
"Why, then, did God give them 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 hardly be 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've got to be free.
"Of course God knew what would happen if they used their freedom the wrong way: apparently, He thought it worth the risk. (...) If God thinks this state of war in the universe a price worth paying for free will --- that is, for making a real world in which creatures can do real good or harm and something of real importance can happen, instead of a toy world which only moves when He pulls the strings --- then we may take it it is worth paying.”