Man oscillates between periods of spiritual soaring and periods of rest from the spirit, when he is content with the simple things of life.
Also, there seems to be some danger in all extremes.
Riches lead to one sort of temptation, poverty to another.
Thus, while the wicked get God's attention, causing Him to occupy Himself with them, the saintly are privileged to suffer as their portion, and the mediocre are usually happy to avoid either extreme and are satisfied to live out their lives in tranquil dullness and insensitivity.
However, since nothing shakes them, there is no thrust forward or awakening to another world.
And indeed, suffering is often "wasted" on the complacently mediocre.
This is a very different sort of thing from the "wasting" of suffering on the saintly who simply do not see it as something evil.
For the conscious person, the gift of suffering comes as a test, an opportunity to learn something and to progress into an acceptance of suffering as a hidden blessing.
The point is that man needs to see suffering as something given to him for his own benefit, whether as instruction or as a bitter medicine.
If it is so hidden that one fails to see it, something very serious and complicated happens to one's relationship with God-a kind of destructive bitterness.
On the other hand, the person who is ready to receive the Divine chastisement is the one who is closest to God, even though He is hidden.
In chastisement, God is closer than in correct or formal relations.
As is true in ordinary human associations, rebuke is a part of genuine cherishing.
–Rabbi Adin Steinsaltz
From "The Meaning of Sadness" in The Long Shorter Way by Rabbi Adin Steinsaltz