Good temper is a mean with respect to anger; the middle state being unnamed, and the extremes almost without a name as well, we place good temper in the middle position, though it inclines towards the deficiency, which is without a name. The excess might called a sort of 'irascibility'. For the passion is anger, while its causes are many and diverse.The man who is angry at the right things and with the right people, and, further, as he ought, when he ought, and as long as he ought, is praised. This will be the good-tempered man, then, since good temper is praised. For the good-tempered man tends to be unperturbed and not to be led by passion, but to be angry in the manner, at the things, and for the length of time, that the rule dictates; but he is thought to err rather in the direction of deficiency; for the good-tempered man is not revengeful, but rather tends to make allowances.
It sometimes surprises students to learn that Aristotle taught that it is right to be angry sometimes, because our upbringing via public school tends to punish any expression of anger or resistance. (Or at least it did in my day; I hear that may not always be true these days.) This is to assert control and order upon a classroom of children who might in fact have some good reasons to be angry, such as being forced to spend their childhood not outside running or hunting or fishing, but having to sit all day memorize multiplication tables.
It is in fact true, though, that it is important to get angry about some things. Getting the right amount of angry at only the right things is the point of balance here. It is wrong to get too angry, or at the wrong things, but also not to get angry enough at the right things, or not to get angry at all.