Objection 1. It would seem that anger does not require an act of reason. For, since anger is a passion, it is in the sensitive appetite. But the sensitive appetite follows an apprehension, not of reason, but of the sensitive faculty. Therefore anger does not require an act of reason.
Objection 2. Further, dumb animals are devoid of reason: and yet they are seen to be angry. Therefore anger does not require an act of reason.
Objection 3. Further, drunkenness fetters the reason; whereas it is conducive to anger. Therefore anger does not require an act of reason.
On the contrary, The Philosopher says (Ethic. vii, 6) that "anger listens to reason somewhat."
I answer that, As stated above (Article 2), anger is a desire for vengeance. Now vengeance implies a comparison between the punishment to be inflicted and the hurt done; wherefore the Philosopher says (Ethic. vii, 6) that "anger, as if it had drawn the inference that it ought to quarrel with such a person, is therefore immediately exasperated." Now to compare and to draw an inference is an act of reason. Therefore anger, in a fashion, requires an act of reason.
Reply to Objection 1. The movement of the appetitive power may follow an act of reason in two ways. In the first way, it follows the reason in so far as the reason commands: and thus the will follows reason, wherefore it is called the rational appetite. In another way, it follows reason in so far as the reason denounces, and thus anger follows reason. For the Philosopher says (De Problem. xxviii, 3) that "anger follows reason, not in obedience to reason's command, but as a result of reason's denouncing the injury." Because the sensitive appetite is subject to the reason, not immediately but through the will.
Reply to Objection 2. Dumb animals have a natural instinct imparted to them by the Divine Reason, in virtue of which they are gifted with movements, both internal and external, like unto rational movements, as stated above (Question 40, Article 3).
Reply to Objection 3. As stated in Ethic. vii, 6, "anger listens somewhat to reason" in so far as reason denounces the injury inflicted, "but listens not perfectly," because it does not observe the rule of reason as to the measure of vengeance. Anger, therefore, requires an act of reason; and yet proves a hindrance to reason. Wherefore the Philosopher says (De Problem. iii, 2,27) that whose who are very drunk, so as to be incapable of the use of reason, do not get angry: but those who are slightly drunk, do get angry, through being still able, though hampered, to form a judgment of reason.
Source: Summa Theologica (New Advent)