Nearly all of us have asked this question at one time or another. The answer hinges on two basic truths: First, most suffering is not due to so-called natural causes. In other words, it's not according to the way God intended for things to work; it's not His doing. It's man's doing. Second, He has given us freedom of choice, and this plays a big part in His plan for us. He didn't create us as robots, but with decision-making abilities and the need to exercise them. He put us here to make choices between right and wrong, good and evil.
So when it comes right down to it, most suffering is the result of people's choices. Sometimes people deliberately inflict suffering on others, and sometimes the suffering is a byproduct of selfish choices or indifference--choosing not to make choices that could avoid or alleviate the suffering. But either way, people are to blame. God doesn't approve of choices that harm others or us, but if He were to step in every time we made a bad choice, He'd have to put an end to freedom of choice altogether.
Why do people make choices that hurt others?--Because they put themselves first and choose their own ways over God and His loving ways. God has given each of us a conscience--an inborn sense of right and wrong. In the Bible He also gives us the solution to suffering and all the other ills we bring upon ourselves. It's a solution that's so simple it can be summed up in one word: love. "Love does no harm to a neighbor" (Romans 13:10). God tries to guide us into doing what's loving and right, but He won't force us.
God is not to blame for all the suffering in the world. He's not some kind of monster that enjoys making people suffer. It's not God who causes the pain, death, and heartache. The truth is, much of what we suffer is caused by people's selfishness and destructive attitudes and actions.