Chabadshlucha – I just read this thread for the first time. I read your OP, and skimmed through the answers.
It seems to me that many of the posters may have been negating the validity of the question. However, I heard the same idea that I think he (and you) are expressing from a well-known Talmid Chacham.
Basically, there are many reasons that can be given for suffering (many of which were given in above posts, I think). These reasons have to do with why it is good for us. But we are still left with the question: Why did Hashem create a system in which we can only gain those things through suffering? Hashem created the rules of (human)nature – He could have created those rules differently in the first place!
The answer is that there is no answer that we can possibly understand, since we ourselves are also within the system, so we can only answer questions from within the system – we can’t answer questions that precede the system (and therefore are outside the system).
In terms of how to satisfy the man, you can tell him this and then add: If you need to take your infant to get shots, how can you explain to an infant why you are handing him over to someone who is going to torture him and stick needles in him if you love him?