Objection 1. It would seem that a greater power should not be reckoned a cause of sorrow. For that which is in the power of the agent is not present but future. But sorrow is for present evil. Therefore a greater power is not a cause of sorrow.
Objection 2. Further, hurt inflicted is the cause of sorrow. But hurt can be inflicted even by a lesser power. Therefore a greater power should not be reckoned as a cause of sorrow.
Objection 3. Further, the interior inclinations of the soul are the causes of the movements of appetite. But a greater power is something external. Therefore it should not be reckoned as a cause of sorrow.
On the contrary, Augustine says (De Nat. Boni xx): "Sorrow in the soul is caused by the will resisting a stronger power: while pain in the body is caused by sense resisting a stronger body."
I answer that, As stated above (Article 1), a present evil, is cause of sorrow or pain, by way of object. Therefore that which is the cause of the evil being present, should be reckoned as causing pain or sorrow. Now it is evident that it is contrary to the inclination of the appetite to be united with a present evil: and whatever is contrary to a thing's inclination does not happen to it save by the action of something stronger. Wherefore Augustine reckons a greater power as being the cause of sorrow.
But it must be noted that if the stronger power goes so far as to transform the contrary inclination into its own inclination there will be no longer repugnance or violence: thus if a stronger agent, by its action on a heavy body, deprives it of its downward tendency, its consequent upward tendency is not violent but natural to it.
Accordingly if some greater power prevail so far as to take away from the will or the sensitive appetite, their respective inclinations, pain or sorrow will not result therefrom; such is the result only when the contrary inclination of the appetite remains. And hence Augustine says (De Nat. Boni xx) that sorrow is caused by the will "resisting a stronger power": for were it not to resist, but to yield by consenting, the result would be not sorrow but pleasure.
Reply to Objection 1. A greater power causes sorrow, as acting not potentially but actually, i.e. by causing the actual presence of the corruptive evil.
Reply to Objection 2. Nothing hinders a power which is not simply greater, from being greater in some respect: and accordingly it is able to inflict some harm. But if it be nowise stronger, it can do no harm at all: wherefore it cannot bring about that which causes sorrow.
Reply to Objection 3. External agents can be the causes of appetitive movements, in so far as they cause the presence of the object: and it is thus that a greater power is reckoned to be the cause of sorrow.
Source: Summa Theologica (New Advent)