Augustine of Hippo

"But because I have said these things unto you," He adds, "sorrow hath filled your heart." He saw, indeed, what effect these words of His were producing in their hearts; for having not yet within them the spiritual consolation, which they were afterwards to have by the Holy Spirit, what they still saw objectively in Christ they were afraid of losing; and because they could have no doubt they were about to lose Him whose announcements were always true, their human feelings were saddened, because their carnal view of Him was to be left …

Tractates on John 94 ↗

Thomas Aquinas

2085 Now he mentions the sorrow of the disciples. For Chrysostom this sorrow is the effect of Christ's prediction of the future troubles of the disciples ‑ For Augustine, their sorrow is the effect of Christ's leaving, for they were glad to be in his presence, and attracted in a certain carnal way to him in his human nature, like one friend is pleased at the presence of another. …

Commentary on John ↗

Bernard of Clairvaux

They could not even bear to hear a word of his approaching passion and death, although this was to be their salvation. Even after it had all happened they could not gaze upon the glory of his ascension without deep sorrow. This is why Christ said to them: "Because I have said this to you sadness has filled your hearts." So it was only by his physical presence that their hearts were detached from carnal loves.

Sermons on the Song of Songs, Sermon 20 ↗