Why does God allow so much evil, pain and suffering in the world? We know instinctively that He could stop it all … but He doesn’t — so we’re tempted to doubt His goodness. What could be His motive?
Well, what would it be like if God never allowed anything bad to happen? Sure, there’d be no sin or suffering, but what would we know about God or ourselves?
We’d never know He was preventing evil and suffering … would we? We’d never 
experience His mercy or patience; we’d know nothing of His sacrificial love or His willingness to suffer with us, or of His justice, wrath and holiness … or of our own selfishness and depravity … and very little of His wisdom and power. It would be pleasant for sure, but rather dull … uninteresting … boring. There’d be no contrast.
The truth is that God is preventing most of the evil that could be occurring in this world (2Th 2:7): no one has ever fully expressed their own depravity. (Je 17:9)
By allowing the evil that He has, God has been perfectly revealing both Himself and everything outside Himself; this is actually His motive in Creation: the evil He allows perfectly reveals both His nature and ours. (Ro 9:22-23) God wants us to experience Him, to know Him intimately in all of His character and holiness. This is only possible as we see Him responding to a wide variety of enemies who are opposed to Him.
Will knowing God intimately be worth it all in the end? Evidently, God thinks so … and He’s already there (Is 57:15) … bringing forth unspeakable beauty from all the brokenness. (Is 61:3, 1Pe 1:7)
The truth is, God hasn’t responded to most of the evil in the world yet, but He will one Day. (Ac 17:31) Just because we haven’t seen full justice doesn’t mean we won’t. And if the little we’ve seen of His response so far is any indication, it will be utterly amazing, glorious beyond description. (Re 20:11)
Meanwhile, God has shown us enough to help us rejoice in Him, to trust Him implicitly and confidently, and to glory in Him alone. (Je 9:23-24) Let’s do so, believing He will never break a promise, be unfaithful, or a disappointment in the end. “Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.” (1Co 2:9)