"The tristitia theme belongs . . . to the most beautiful [of] expositions in the [treatise on] Christology of Thomas. . . . Christ wants to experience sadness [(die Traurigkeit)] in the face of death, first, because it belongs to the proper attitude with respect to [(Einstellung gegenüber)] death; second, in order to suffer and thus to redeem us; and, finally, in order to provide us with an example (cf. how [in the commentary] In Joh. Thomas turns even the natural sense of the injunction 'Non turbetur cor vestrum [(Let not your heart be troubled)]' completely into its opposite)."
Leo Scheffczyk on ST III.14-15, in "Die Stellung des Thomas von Aquin in der Entwicklung der Lehre von den mysteria vitae Christi," in Renovatio et reformatio: wider das Bild vom "finsteren" Mittelalter: Festschrift für Ludwig Hödl zum 60. Geburtstag überreicht von Freunden sowie Kollegen und Schülern, ed. Manfred Gerwing und Godehard Ruppert (Münster: Aschendorff, 1985), 60 (44-70).
The passage in the commentary In Joh. to which Scheffczyk refers is this one: "Here he is not forbidding them to be troubled in spirit, but he is forbidding that their hearts, that is, their spirits, be troubled. For there is a troubled state which arises from reason; this is to be praised and is not forbidden" (trans. Joseph Kenny).