I'm asked that quite often, as I deal with sick people all the time.
I usually start out with, "I don't know why God allows bad things to happen to good people." And I go from there with whatever comments they may make.
I don't brush off their frustrations or even anger at God for their circumstances, but I do tell them that sometimes, while life isn't fair, there can be a peace given to us in our current situation. When something so devastating happens, as you mention above, there's nothing we can do but move on. I don't go about it that bluntly, but over the course of a conversation, that's the point that's generally made. In addition, I let them know that sometimes in our sufferings, we can help others, and in turn, help ourselves.
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I've gone and done it now! I'm on Facebook!!!