I know, this sounds ridiculous and contradictory but let me explain. As children, we learn what our bodies were not made for through pain (our hands were not made for the stove, etc.). In a similar way, God is trying to teach us as spiritual children that we were not made for this world. More precisely, we were not made primarily for any desire in this world but rather were made to desire God above all else.
This, in fact, is how the whole pain thing started. In the garden, man and woman rejected God as their supreme desire and chose the desire for autonomy and freedom from God (in the form of choosing the forbidden fruit) instead. Through this act, not only did man sin and become fallen, but the world was subjected to bondage to corruption and futility. Because of this, creation has not functioned in the perfect harmony that it was intended for.
Why? Why did God do this to the world? Why so many children with heart-wrenching disabilities? Why natural disasters that take so many lives? Why famines and diseases? As John Piper puts it, God put the world under a curse so that the physical horror we see around us would become a vivid picture of how horrible sin is. In other words, natural evil is a signpost to the unspeakable wickedness of moral evil. God allowed the disorder of the natural world to match the disorder of the moral and spiritual world. Diseases and deformities are God's portraits of what sin is like in the spiritual realm and that is true even though some of the most Godly people bear the most horrible deformities.
Yet we don't feel it! In our present, fallen condition our hearts are so numb and so blinded we seldom feel the gravity of our sin. Almost no one feels the abhorrence that sin is or feels repulsed or nauseated at how they scorn the glory of God. We should feel as deeply about sin as we do about a friend's disability. We should feel as intense and bad about our immorality as we do about starvation. O, that we could feel how offensive and repugnant and abominable it is to prefer anything to your Maker! To plagiarize John Piper once more, the natural world is shot through with horrors that aim to wake us up from the dream world of thinking demeaning God is no big deal.
Thus, if there were not the pain that there now is, it would be far too easy to forget God and how we have spurned Him. It would be far too easy to prefer all the temporary pleasures that this world offers rather than prefer the eternal joy in Christ. To quote C.S. Lewis, "We are half-hearted creatures like an ignorant child wants to go making mud pies in a slum because he cannot imagine what is meant by the offer of a holiday at the beach. We are far too easily pleased."