Donald S. Whitney, Spiritual Disciplines for the Christian Life, 1991, p. 81.
Sometimes a failure to persist in prayer proves that we were not serious about our request in the first place. At other times God wants us to persist in prayer in order to strengthen our faith in Him. Faith would never grow if all prayers were answered immediately. Persistent prayer tends to develop deeper gratitude as well. As the joy of a baby’s birth is greater because of the months of anticipation, so is the joy of an answer to prayer after persistent praying. And as much as a generation that measures time in nanoseconds hates to admit its need for it, God crafts Christlike patience in us when He requires persistence in prayer.
Donald S. Whitney, Spiritual Disciplines for the Christian Life, 1991, p. 81.