Allah and His Creation...

God created suffering and heartache, so that joy might be known as their opposite.
Hidden things become manifest through their opposites.
But God has no opposite; so he remains hidden.
Light is known as the opposite of darkness.
But God's light has no opposite.
Thus we cannot know him through our eyes.

-Rumi, "Mathnawi"

Learning About Ourselves...

People differ greatly in the amount of anxiety and worry they can stand: some can endure a great deal without being adversely affected, while others soon show signs of strain.
You should observe yourself closely, so that you understand your own powers of endurance.
By this means you will learn to divest yourself before any harm is done.

-Razi: Kitab al-Muluki