I answer that, All the causes of anger are reduced to slight. For slight is of three kinds, as stated in Rhet. ii, 2, viz. "contempt," "despiteful treatment," i.e. hindering one from doing one's will, and "insolence": and all motives of anger are reduced to these three. Two reasons may be assigned for this. First, because anger seeks another's hurt as being a means of just vengeance: wherefore it seeks vengeance in so far as it seems just. Now just vengeance is taken only for that which is done unjustly; hence that which provokes anger is always something considered in the light of an injustice. Wherefore the Philosopher says (Rhet. ii, 3) that "men are not angry---if they think they have wronged some one and are suffering justly on that account; because there is no anger at what is just." Now injury is done to another in three ways: namely, through ignorance, through passion, and through choice. Then, most of all, a man does an injustice, when he does an injury from choice, on purpose, or from deliberate malice, as stated in Ethic. v, 8. Wherefore we are most of all angry with those who, in our opinion, have hurt us on purpose. For if we think that some one has done us an injury through ignorance or through passion, either we are not angry with them at all, or very much less: since to do anything through ignorance or through passion takes away from the notion of injury, and to a certain extent calls for mercy and forgiveness. Those, on the other hand, who do an injury on purpose, seem to sin from contempt; wherefore we are angry with them most of all. Hence the Philosopher says (Rhet. ii, 3) that "we are either not angry at all, or not very angry with those who have acted through anger, because they do not seem to have acted slightingly."
The second reason is because a slight is opposed to a man's excellence: because "men think little of things that are not worth much ado" (Rhet. ii, 2). Now we seek for some kind of excellence from all our goods. Consequently whatever injury is inflicted on us, in so far as it is derogatory to our excellence, seems to savor of a slight.