The Messenger of Allah (may Allah bless him and grant him peace) said, "Had this world been worth a mosquito wing in the Sight of Allah, He would not have given a disbeliever from it a sip of water."
Linguistically, Ad-Du‘â’ [the Arabic word for supplication, invocation, prayer….] is a call, and it has more than a denotation. With respect to humans, if addressed from a superior to an inferior it would be a command: as when the boss asks his employee to execute such and such tasks. That would be an order. It could be from a human to another co-equal to him, thus it would be a request. It could be from an employee to his boss; in such case it would be a wish.
Ad-Du‘â’ according to the terminology of the Sharî‘ah [Islamic Law] is a request from a servant to His Lord. Supplication dictates that there must be a supplicant (the servant) and a highest Answerer (Allah), all praise and glory be to Him. The purpose of supplication is having (what man begs for). Allah, Blessed and Exalted be He, has strewn His Causes in His Universe for the believer and the non-believer to adopt them. Therefore, supplication should be addressed to The Causer not the causes. In his supplication man begs for things he sees as good for him, judged by his limited knowledge, imperfect human power and imbalanced criteria.
But Allah, glory be to Him – Who alone possesses Power and Knowledge – sees that the thing His servant unknowingly begs for is evil for him and the Mercy lies here in not answering the servant’s call, so the unfulfillment of the prayer becomes the bestowed good and the given response.
We may clarify the issue and approximate the picture to the minds by a son who asks his father to buy him a gun in order to protect himself. The son imagines that having a gun is good for him, because people will fear him and he will be abler to defend himself. Considering the request from the son’s viewpoint, we may see that his demand will avail him good, but actually it will not. For the son may act rashly and rushes into a dispute or a fight, looses his temper, shoots his opponent and commits a crime against a human soul. He may also incur his own death, thus forbidding him to carry a gun is for his own good. But the son sees the unfulfillment of his demand as a denial of being a fear-inspiring man, capable of defending himself. The son is blind to the wisdom behind the unfulfillment, but the father knows where lies the good and he acted accordingly.
Man should invoke Allah for whatever good he may wish for, but likewise he must regard the denial as in his favour. He has to leave the answer that will surely bring him good only to Allah, whether the answer is by fulfillment or denial, and not to distress himself by his impatience for not being answered.