6 But by this line of argument we have shown why Christ loved John more than Peter, not why Peter loved Christ more than John. For if Christ loves us more in the world to come, where we shall live unendingly with Him, than in the present, from which we are in the course of being rescued, that we may be always in the other, it does not follow on that account that we shall love Him less when better ourselves; since we can in no possible way be better ourselves, save by loving Him more.
Why was it, then, that John loved Him less than Peter, if he signified that life, wherein He must be more abundantly loved, but because on that very account it was said, “I will that he tarry,” that is wait, “till I come;” for we have not yet the love itself, which will then be greater far, but are expecting that future, that we may have it when He shall come? Just as in his own epistle the same apostle declares, “It has not yet appeared what we shall be: but we know that, when He shall appear, we shall be like Him; for we shall see Him as He is.” Then accordingly shall we love the more that which we shall see.
But the Lord Himself, in His predestinating knowledge, loves more that future life of ours that is yet to come, such as He knows it will be hereafter in us, in order that by so loving us He may draw us onward to its possession. Wherefore, as all the ways of the Lord are mercy and truth, we know our present misery, because we feel it; and therefore we love more the mercy of the Lord, which we wish to be exhibited in our deliverance from misery, and we ask and experience it daily, especially in the remission of sins: this it is that was signified by Peter, as loving more, but less beloved; because Christ loves us less in our misery than in our blessedness.
But the contemplation of the truth, such as it then shall be, we love less, because as yet we neither know nor possess it: this was signified by John as loving less, and therefore waiting both for that state itself, and for the perfecting in us of that love to Him, to which He is entitled, till the Lord come; but loved the more, because that it is, which is symbolized by him, that makes him blessed.
Source: Tractates on the Gospel of John (New Advent)