And, instead of an immediate response to perceived need, prayer becomes more like a last resort. Prayer is regarded as a desperate course of action precisely because its results seem unpredictable. A good many people who pray, do so with this lack of expectation in the back of the mind. Maybe it's a means of getting God off the hook, or maybe it's an acknowledgement that some of our petitions will cancel out others, like vacationers dueling with farmers for sunny and rainy weather all at once. It is as easy to be sympathetic to that approach as it is to the first.
Expectation is a very difficult thing to sustain in prayer.
At least some of our experience suggests that explanations like those I've just offered are more reasonable than a stubborn acceptance that we will find our prayers answered. And this is how the disciples felt, too, I'm sure. That's why Jesus more than once offers up stories and parables extolling stubborn, expectant prayer. And, although Jesus offers a prayer to pray, he doesn't go into a lot of theory about what constitutes a right or wrong, or legitimate or illegitimate element of prayer. It is left open-ended.