I read this in Tell It Slant by Eugene Peterson (He is quoting Evagrius the Solitary from The Philokalia):

Often when I have prayed I have asked for what I thought was good, and persisted in my petition, stupidly importuning the will of God, and not leaving it to Him to arrange things as he knows is best for me. But when I have obtained what I asked for, I have been very sorry that I did not ask for the will of God to be done; because the thing turned out not to be as I had thought.... Do not be distressed if you do not at once receive from God what you ask. He wishes to give you something better-to make you persevere in your prayer. For what is better than to enjoy the love of God and to be in communion with Him?