61 “And whosoever,” says He, “shall compel you to go a mile, go with him other two.” And this, certainly, not so much in the sense that you should do it on foot, as that you should be prepared in mind to do it. For in the Christian history itself, which is authoritative, you will find no such thing done by the saints, or by the Lord Himself when in His human nature, which He condescended to assume, He was showing us an example of how to live; while at the same time, in almost all places, you will find them prepared to bear with equanimity whatever may have been wickedly forced upon them.
But are we to suppose it is said for the sake of the mere expression, “Go with him other two;” or did He rather wish that three should be completed—the number which has the meaning of perfection; so that every one should remember when he does this, that he is fulfilling perfect righteousness by compassionately bearing the infirmities of those whom he wishes to be made whole? It may seem for this reason also that He has recommended these precepts by three examples: of which the first is, if any one shall smite you on the cheek; the second, if any one shall wish to take away your coat; the third, if any one shall compel you to go a mile: in which third example twice as much is added to the original unit, so that in this way the triplet is completed.
And if this number in the passage before us does not, as has been said, mean perfection, let this be understood, that in laying down His precepts, as it were beginning with what is more tolerable, He has gradually gone on, until He has reached as far as the enduring of twice as much more. For, in the first place, He wished the other cheek to be presented when the right had been smitten, so that you may be prepared to bear less than you have borne. For whatever the right means, it is at least something more dear than that which is meant by the left; and if one who has borne with something in what is more dear, bears with it in what is less dear, it is something less.
Then, secondly, in the case of one who wishes to take away a coat, He enjoins that the garment also should be given up to him: which is either just as much, or not much more; not, however, twice as much. In the third place, with respect to the mile, to which He says that two miles are to be added, He enjoins that you should bear with even twice as much more: thus signifying that whether it be somewhat less than the original demand, or just as much, or more, that any wicked man shall wish to take from you, it is to be borne with tranquil mind.
Source: Our Lord's Sermon on the Mount (New Advent)