Excerpts taken from Salvifici Doloris Letter of Pope John Paul II on the Christian Meaning of Human Suffering, 11 February 1984 http://www.ewtn.com/faith/teachings/rdmpc1.htm
All in the animal world expresses pain but only man knows he is suffering and wonders, why? And if he doesn't have an answer, he suffers all the more.
Why is there "suffering" and "evil"?
These are difficult questions.
Man does not ask the world these questions, he puts this question to God, with all the emotion of his heart!
God expects this question and listens to it as in the story of Job in the Bible. Job was a holy man that had terrible things happen to him. Even God said Job was innocent.
Job's suffering is the suffering of someone who is innocent and here, it must be accepted as a mystery, which man is not able to understand.
Yes, suffering is a punishment for sin, but not all suffering has the nature of a punishment. God can also be testing us.
"It is suffering, more than anything else, which clears the way for the grace, which transforms human souls."
"Our Lord crucified you so that you do not run away and abandon Him and so that you remain always faithful to Him."
Secret Of The Cross