Why do bad things happen to good people? Why is the world so F%&^ed up? Why does God want there to be acne, evil dictators, global warming, bad television? Lets face it- He's not doing a great job.
Here are the qualities of God as He is traditionally reckoned:
* Omniscient (all knowing)
* Omnipotent (all powerful)
* Benevolent (all good)
The existence of evil requires that one of these 3 is not true BUT if this is the case, it pretty much excludes God from being God by definition (reductio ad absurdum). God and 'evil' cannot logically coexist.
One might argue that we mere mortals cannot understand why (for example) the birth of a severely retarded child is in God's best interests. Great minds have contorted themselves through the ages and arrived at a number of complex theoretical structures to explain away the problem of evil. Many require one to believe a large number of other unbelievable things in order to square the circle.
I prefer Laplace's comments (to Napoleon) when asked whether he thought God existed he said: "I have no need for that hypothesis.".