Whoever fills his heart with satisfaction regarding the Divine Decree, Allah fills his heart with richness, safety, and comfort. And whoever becomes dissatisfied, then his heart will be filled with the opposite, and his heart will be preoccupied with matters that clash with happiness and success.
Therefore contentment empties the heart of all superfluous carriage, thus leaving it entirely for Allah. Discontent removes from the heart all thought of Allah. And so there is no real life for the resentful, complaining person who always feels that he is shifting from one problem to the next. He feels his sustenance to be insufficient, his luck poor, his problems manifold, and above all, he feels that he is deserving of more. Basically, he is discontented with what Allah decreed for him. How then can such a person find comfort, peace, and a good life?
(That is because they followed that which angered Allah, and hated that which pleased Him. So He made their deeds fruitless.) (Quran 47: 28)