The reason he uses that he does not know the purpose of suffering because we dont know the plan that god has can be used in other areas as well, like when reading the koran, there is no way of knowing the full meaning of that book because it is always an interpretation that we have to make. So when you dont know the full context or the reasons for the interpretations, then who do you trust? The actual fact is that you trust in yourself, not god. This does not have to be a bad thing, it just means that you are responsible for the good things that you do.