Why the sad things are said

A year ago John Piper was preaching from John 12:37-50, containing what Piper contends are some of the saddest, most painful parts of the Gospel of John.  And after listening to his exposition, it's hard to disagree with his analysis.
However, in his introductory comments, Dr Piper gives a most helpful and illuminating insight as to why the sad things are said.

"The last thing the aged Simeon said to Mary when he blessed Jesus was this:
Behold, this child is appointed for the fall and rising of many in Israel, and for a sign that is opposed (and a sword will pierce through your own soul also), so that thoughts from many hearts may be revealed. (Luke 2:34–35)
"In other words, it will not all be happy. The truth in many hearts will be revealed. Jesus will be opposed. Many will rise because of him. And many will fall. And a sword will go through Mary’s heart. Many will fall because of Jesus. 
"But make sure that you know and embrace this truth: Jesus and John don’t tell us sad things to leave us in sadness. Make sure you embrace that. They tell us sad things, in the end to make us glad.

• The dark things in the Bible are spoken for the sake of light.
• The ugly things in the Bible are spoken for the sake of beauty.
• The painful things in the Bible are spoken for the sake of comfort.
• The sorrowful things in the Bible are spoken for the sake of joy.
• And conflict in the Bible is pictured for the sake of peace.

"We know this, because Jesus said in John 15:11 that he speaks his words so that our joy might be full, and many of those words were painful. And John says in John 20:31 that the reason he wrote his Gospel was that we might have eternal life;  and many of those words that are painful are aimed at life. So we know those words of Jesus and the stories of John have dark and sad things in them. But we know those dark, sad things are for our joy and for our life."