Luther said that he could never rightly understand some of the Psalms, till he was in affliction. Affliction teaches what sin is. In the word preached, we hear what a dreadful thing sin is, that it is both defiling and damning, but we fear it no more than a painted lion; therefore God lets loose affliction, and then we feel sin bitter in the fruit of it. A sick-bed often teaches more than a sermon. We can best see the ugly visage of sin in the glass of affliction. Affliction teaches us to know ourselves. In prosperity we are for the most part strangers to ourselves. God makes us know affliction, that we may better know ourselves. We see that corruption in our hearts in the time of affliction, which we would not believe was there. Water in the glass looks clear, but set it on the fire, and the scum boils up. In prosperity, a man seems to be humble and thankful, the water looks clear; but set this man a little on the fire of affliction, and the scum boils up — much impatience and unbelief appear. “Oh,” says a Christian, “I never thought I had such a bad heart, as now I see I have; I never thought my corruptions had been so strong, and my graces so weak.
Thomas Watson, A Divine Cordial (Kindle Edition, location 220)