During hard times, if one remains contented with one’s situation, one is able to remain calm and composed.
In contrast, the malcontented heart is filled with sickness, doubt, and instability; such a heart remains rebellious and troubled. It is as if this heart too has a voice, but the words it speaks are ever so different.
The people who possess this kind of heart are contradictory in their dealings. If they have rights upon another, they hurry to claim their rights. Yet if they are sought after to fulfill their obligations, they turn away in disdain. When good befalls them they feel calm and complacent, but when they are put to the test, how sudden is their change for the worse! They are those who have lost not only this world, but also the Hereafter.
So those are two opposites which yield results that are equally opposite in nature: contentment leads to peace, which eventually leads to prosperity; resentment about GOD’S decree leads to anxiety, which eventually leads to failure. You must remember that from the greatest of blessings that GOD can give to one of his slaves is bestowing tranquility upon him, and one of the best ways of achieving tranquility is being contented and pleased with GOD at all times.