Some years ago my wife was scheduled to have major surgery.  The subject was a delicate one, and I was not entirely convinced of the merits.  We therefore asked the doctors for a second opinion from a specialist.  This doctor replied, in the jargon of the medical profession, that "this particular surgery is indicated as mandatory."  That phrase was supposed to relieve my anxieties, but it didn't.  Surgery is a friendly attack with a knife, and major surgery should not be performed at the doctor's whim.
The solution was provided by my own doctor.  I went to him for a routine examination.  He was an extremely polite and proper individual, gifted with the ability to deal with a delicate situation. 
"I perceive that you are having some difficulty in accepting your wife's surgery.  May I ask what is troubling you?"
"Well, doc, I'd just like to have someone explain to me, in plain English and not medical talk, why we're doing this."
"Ah!  It is simple.  If we don't, some day she'll bleed to death."
That put the matter in a different perspective.  Once I knew why it was necessary, I had no further objections.
The disciples, I expect, had the same problem here.  They knew, as we do, that the burden of the future is lighter if you know what is going to happen and why.  Here Christ tries to explain it to them--including his betrayal as well as his death--but the Scripture tells us that the meaning was hidden from them.  And just as I had a socially trained reluctance to inquire about the surgery, so they had a reluctance to inquire about death.
The matter was hidden from them, I suspect, so that they would not bear any guilt in the matter.  Had they known who would betray Jesus, it would have burdened their consciences later.
There is good news in this too.  If the foreknowledge of dangerous and grim things lightens them, then what about the foreknowledge of great and glorious things?  Does it not also change the way we should think?  Should we not view our sufferings now with a lighter heart, knowing what great things are to come? We know that some day our Lord will return, and at his return the dead shall rise from the grave.  We know that at his return to judge the living and the dead the righteous shall receive what is their due, and that we shall live and reign with him.  What a glorious hope! 
Therefore, in the meanwhile, let us not worry about tomorrow's pains and sufferings, but keep our eyes on our Lord, working until he returns in glory.