Pain in the Big Picture

Human suffering is such a staggering thing to wrap one's mind around. Why, in the big picture, does it exist? Do we need it? Why do we hurt?
Though I think the conclusion is a hasty one, I can understand the feelings motivating people to accept the atheist conclusion of the Problem of Evil. (To read my critique of the problem of evil, click here.) I can also understand how the pathos of human suffering might evoke a reaction of obstinate agnosticism. Sadness, in whatever form it is delivered or received, however, seems to evoke sincere thoughts about the more vital aspects of life and existence. Sadness causes us to reevaluate our lives, our behaviors and the way we view life.
Physiologically speaking, pain is there to warn us of danger and keep us from seriously hurting ourselves. You could, in some rudimentary way, say that our body is trying to teach us, by example, about the dangers in the world around us. Pain, in this sense, is a tool by which we are able to understand what is safe and what is not.
I think it fair to say that the same definition could apply to sadness. What is sadness, but emotional pain? Sadness, in my experience, has been the tool by which I have more clearly defined the happiness in my life. We tend to quickly grow accustomed to our privileges and the good things in our lives. Sadness is the standard by which we measure the good in our lives. It is the framework, so to speak, on which we hang our understanding of the good and bad in our universe.