Do not revile the fever, for it cleanses out the sins of the children of Adam in the same way that a furnace removes the dirt of iron​

This Hadith teaches us to be polite with Allah the Almighty, we may not utter a word that indicates dissatisfaction with fate, even indirectly. One may not ever utter any word that entails despair. It would have a negative effect on those around him who may reject his words or hate him. It would be worse if the listener is a flunkey who imitates this falsehood.