5 But what mean the words, “whom Jesus loved”? As if He did not love the others, of whom this same John has said above, “He loved them to the end”; and as the Lord Himself, “Greater love has no man than this, that a man lay down his life for his friends.” And who could enumerate all the testimonies of the sacred pages, in which the Lord Jesus is exhibited as the lover, not only of this one, or of those who were then around Him, but of such also as were to be His members in the distant future, and of His universal Church? But there is some truth, doubtless, underlying these words, and having reference to the bosom on which the narrator was leaning. For what else can be in dicated by the bosom but some hidden truth? But there is another more suitable passage, where the Lord may enable us to say something about this secret that may prove sufficient.
Source: Tractates on the Gospel of John (New Advent)