I am going to address the question in terms of emotional pain, because the role of physical pain is a matter of science and physical health, not really a literary, psychological, or philosophical matter.
So, why do we need emotional pain and difficulty in our lives? I believe that if we were always happy and everything were easy, we would not know we were happy or how good we had it, and I also believe we would not grow, within ourselves, in our relationships with others, or in our efforts to make a better world.
In The Giver, you will notice that drugs are necessary to inhibit people's pain and the rules are meant to do away with difficulty, particularly the difficulties of choice and inequity. We can infer that one of the author's points is that these are a natural part of being human and that we interfere with these natural conditions at our peril.
Why would we ever do anything...
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