You may still want to ask why the Apostle said: We do not know what it is right to pray for, because, surely, we cannot believe that either he or to those to whom he wrote did not know the Lord's Prayer..
He showed that he himself shared this uncertainty. Did he know what it is right to pray for when he was given a thorn in the flesh, an angel of Satan to bruise him, so that he might not be puffed up by the greatness of what was revealed to him? Three times he asked the Lord to take it away from him, which showed that he did not know what he should ask for in prayer. At last, he heard the Lord's answer, explaining why the prayer of so great a man was not granted, and why it was not expedient for it to be granted: My grace is sufficient for you, for power shines forth more perfectly in weakness.
In the kind of affliction, then, which can bring either good or ill, we do not know what it is right to pray for; yet, because it is difficult, troublesome and against the grain for us, weak as we are, we do what every human would do, we pray that it may be taken away from us. We owe, however, at least this much in our duty to God: if he does not take it away, we must not imagine that we are being forgotten by him but, because of our loving endurance of evil, must await greater blessings in its place. In this way, power shines forth more perfectly in weakness. These words are written to prevent us from having too great an opinion of ourselves if our prayer is granted, when we are impatient in asking for something that would be better not to receive; and to prevent us from being dejected, and distrustful of God's mercy toward us, if our prayer is not granted, when we ask for something that would bring us greater affliction, or completely ruin us through the corrupting influence of prosperity. In these cases we do not know what it is right to ask for in prayer.
Therefore, if something happens that we did not pray for, we must have no doubt at all that what God wants is more expedient than what we wanted ourselves. Our great Mediator gave us an example of this. After he had said: Father, if it is possible, let this cup be taken away from me, he immediately added, Yet not what I will, but what you will, Father, so trnasforming the human will that was his through his taking a human nature. As a consequence, and rightly so, through the obedience of one man the many are made righteous.
From a letter to Proba by Saint Augustine, bishop. We do not know what is right to pray for. Taken from The Liturgy of the Hours, Volume IV: Ordinary Time - Weeks 18-34, Twenty-Ninth Week in Ordinary Time, Office of Readings, pp. 425-426. (Ep. 130, 14, 25-26; CSEL, 44, 68-71)