Objection 1. It would seem that the human will in Christ did not will anything except what God willed. For it is written (Psalm 39:9) in the person of Christ: "That I should do Thy will: O my God, I have desired it." Now he who desires to do another's will, wills what the other wills. Hence it seems that Christ's human will willed nothing but what was willed by His Divine will.
Objection 2. Further, Christ's soul had most perfect charity, which, indeed, surpasses the comprehension of all our knowledge, according to Ephesians 3:19, "the charity of Christ, which surpasseth all knowledge." Now charity makes men will what God wills; hence the Philosopher says (Ethic. ix, 4) that one mark of friendship is "to will and choose the same." Therefore the human will in Christ willed nothing else than was willed by His Divine will.
Objection 3. Further, Christ was a true comprehensor. But the Saints who are comprehensors in heaven will only what God wills, otherwise they would not be happy, because they would not obtain whatever they will, for "blessed is he who has what he wills, and wills nothing amiss," as Augustine says (De Trin. xiii, 5). Hence in His human will Christ wills nothing else than does the Divine will.
On the contrary, Augustine says (Contra Maxim. ii, 20): "When Christ says 'Not what I will, but what Thou wilt' He shows Himself to have willed something else than did His Father; and this could only have been by His human heart, since He did not transfigure our weakness into His Divine but into His human will."
I answer that, As was said (2,3), in Christ according to His human nature there is a twofold will, viz. the will of sensuality, which is called will by participation, and the rational will, whether considered after the manner of nature, or after the manner of reason. Now it was said above (13, 3, ad 1; 14, 1, ad 2) that by a certain dispensation the Son of God before His Passion "allowed His flesh to do and suffer what belonged to it." And in like manner He allowed all the powers of His soul to do what belonged to them. Now it is clear that the will of sensuality naturally shrinks from sensible pains and bodily hurt. In like manner, the will as nature turns from what is against nature and what is evil in itself, as death and the like; yet the will as reason may at time choose these things in relation to an end, as in a mere man the sensuality and the will absolutely considered shrink from burning, which, nevertheless, the will as reason may choose for the sake of health. Now it was the will of God that Christ should undergo pain, suffering, and death, not that these of themselves were willed by God, but for the sake of man's salvation. Hence it is plain that in His will of sensuality and in His rational will considered as nature, Christ could will what God did not; but in His will as reason He always willed the same as God, which appears from what He says (Matthew 26:39): "Not as I will, but as Thou wilt." For He willed in His reason that the Divine will should be fulfilled although He said that He willed something else by another will.
Reply to Objection 1. By His rational will Christ willed the Divine will to be fulfilled; but not by His will of sensuality, the movement of which does not extend to the will of God--nor by His will considered as nature which regards things absolutely considered and not in relation to the Divine will.
Reply to Objection 2. The conformity of the human will to the Divine regards the will of reason: according to which the wills even of friends agree, inasmuch as reason considers something willed in its relation to the will of a friend.
Reply to Objection 3. Christ was at once comprehensor and wayfarer, inasmuch as He was enjoying God in His mind and had a passible body. Hence things repugnant to His natural will and to His sensitive appetite could happen to Him in His passible flesh.
Source: Summa Theologica (New Advent)