This is probably the biggest question, and the biggest obstacle to trusting God, in Christianity. It's a legitimate question, and it deserves a thoughtful answer that honors the amount of pain attached to it. Disclosure: I am writing this while beset by the most physical pain I've experienced since post-polio syndrome started attacking my body with the "unholy trinity" of pain, weakness and fatigue. It hurts to stand, it hurts to walk. Every single step.
Why does God allow it? And my pain is nothing compared to the horrific suffering of millions around the world. Doesn't He care? Why doesn't He stop it—surely He can. He could stop it all with a single word. So why does He let innocent people—especially children, for heaven's sake—suffer?
We need to put evil and suffering into perspective, and that means the Really Big Picture. Starting before the beginning of time. When all there was, was God: Father, Son and Spirit, engaged in a three-Personed "holy hug" that had no beginning and has no end. A continual celebration of love, adoration, respect, and delight in each other. At some point Father God decided to create mankind and draw us into His circle of love, adopting us as sons (Eph. 1:4-5) and creating a Bride for His eternal Son (Rev. 19:7), a fit companion who would reign with the Lamb (Rev. 22:5).
But God knew that all of human history would unfold between the bookends of the creation of mankind and the Marriage Feast of the Lamb. The God of light and life, of love and truth, knew that all those things are found only in Him; He knew that to reject Him meant choosing darkness and death, isolation and deception. He knew that Adam would rebel, that His perfect creation would crash and burn in the Fall, and that everything would be infected and corrupted by sin. He knew that every human being would be born with a compulsion to reject Him, to live disconnected from Him, independent from Him—something like spiritual HIV+, insuring a death sentence. And sure enough, the mortality rate is still 100%.
God knew all this, and He created us anyway. Because He knew the end result was worth it.
Because God is love, He created people to love, and He created people to love Him back. In order for us to choose to return His love, we needed to be free to choose NOT to love Him. God made us with the very real option to say no to Him, so that our yes would mean something. The alternative would be the equivalent to making a phone say, "Good morning, I love you." The words might be there but there is no heart and no choice behind them—they are nothing more than the result of a programming code. God wanted real and actual love, and that meant that some people He made and dearly loved, could and would say no.
When people say no to God, they not only cut themselves off from relationship with Him, they open the door to all kinds of evil. Some of it comes from sinful human hearts; some of it comes from the demonic realm, angels who also said no to God and became devils. Evil was unleashed by Adam when he disobeyed God in the Garden of Eden (Gen. 3) and it has been causing havoc, pain and suffering ever since. Sometimes we need to remind ourselves that this world plagued by pain and disease, deliberate meanness and selfishness, is not God's original perfect creation. If it were, God would indeed be a horrible monster. He knew Adam would open the door to all kinds of evil and suffering, and He allowed Adam to do it anyway. Because He knew the end result was worth it.
Why does God let people suffer?
God uses suffering to cleanse us, to mature us, to burn up shallowness. (Please see my article The Value of Suffering.) He uses pain as His instrument to shape us into the image of His Son (Rom. 8:28-29). God has no magic wand that instantly transforms us from something broken and dirty (and we are far more broken and dirty than we have any idea) into something whole and beautiful. There is no divine "Bibbity-Bobbity-Boo."
Instead, the Son left heaven, wrapped Himself in human flesh, and came to earth where He lived a perfect, sinless life. Every day of His earthly life, He suffered as a human, limiting Himself to a body that would get tired, hungry, thirsty and dirty. What the first Adam messed up, Jesus the Second Adam corrected. Where Adam disobeyed the Father, Jesus learned obedience through suffering (Heb. 5:8). Jesus suffered throughout His incarnation simply because of His limitations as a human, then suffered an unimaginably horrible death through crucifixion, made even worse because He absorbed all the sin of every human being who had ever lived, was living on the earth at that time, and would ever exist in the future. He took our sin into Himself, actually becoming our sin (2 Cor. 5:21), so that when He died, our sin died with Him. But the Father raised Him from the dead, and He is alive at His Father's right hand right now in heaven.
This means that God knows what it means to suffer. There is no pain, no suffering we can endure, that God Himself did not experience even more during Jesus' time on earth. This same suffering God promised, "Behold, I am making all things new" (Rev. 21:5). The Father knew He would send the Son to suffer, and the Son knew that's what He would leave heaven for.
He did it anyway. Because He knew the end result was worth it.
God allows pain and suffering and evil because He has a plan, and He's working His plan. The end result is that He is redeeming and restoring all the evil, pain and suffering of this sin-sick world. He will set all things right in the end. The last chapter of the Bible makes it clear that there is a happy ending to what is NOT a fairy tale. What started out as a Three-Personed holy hug of the Father, Son and Spirit loving each other while still remaining one God, will be a hugely enlarged circle of love that includes millions, possibly billions of people God made in His image, marked "Mine," and drew into the divine circle to love and be loved forever.
At that point I believe we will agree, as we look back on evil, pain and suffering on earth, that it was so, so worth it.