This is a popular question among non-believers and a reason why many claim there is no God. Or, why they believe in another God or religion such as Buddhism which explains this away as "karma". But as a Christian, what do you think is the reason God allows suffering?
Because Adam and Eve didn't appreciate the goodness of what God gave them, nor did they appreciate the good state in which they were created. And let's face the fact that since Adam was our perfect representative, that we all would have done the same thing had we been in his (or Eve's) place. So when it was whispered in their ear that they could have more, they wanted it.
This is why we gripe over every little thing we think goes wrong. This world is full of gripers who are never satisfied or content with who they are or what they are given. We've all done it, because we lack faith that God is in control and plans the best for us. Everyone who gripes is self-centered, and that's why we have so many conflicting "free wills" that cause so much suffering between us. People want more power, more control, more wealth, more comfort, more laziness, more entertainment, more fun, more excitement, more etc.
Paul wrote: 2 Co 4:16-18 "
Therefore we do not lose heart, but though our outer man is decaying, yet our inner man is being renewed day by day. For momentary, light affliction is producing for us an eternal weight of glory far beyond all comparison, while we look not at the things which are seen, but at the things which are not seen; for the things which are seen are temporal, but the things which are not seen are eternal."
Notice he says "momentary light affliction." This is the one who was beaten, whipped, stoned, shipwrecked, snake bitten, hated, threatened, and rejected by his own people. He says it can't even be compared to eternal glory. We need the eternal perspective to appreciate suffering.
And since God didn't have a "Plan B," God meant for us to experience suffering in this world to teach us faith in Him. Suffering causes the glory of God to shine the greatest, because it causes love to shine the greatest, and causes faith to shine the greatest. Therefore, this world full of suffering is the best possible world that could be created for us.
TD