Poetics by Aristotle: we must not demand of Tragedy any and every kind of pleasure, but only that which is proper to it ....
In Aristotle's Ethics (VII.11-14 and X.1-5), he describes pleasure as (1) necessary to human life, (2) not solely derived from the senses, and (3) the outgrowth or product of excellent activity, and (4) not something to be sought deliberately as it is a product not an originating cause. Having said this, compare Aristotle's idea of pleasure to our usual contemporary understanding of pleasure:
American Heritage Dictionary
1. The state or feeling of being pleased or gratified.
2. A source of enjoyment or delight
While there is nothing here to contradict Aristotle, our contemporary understanding of pleasure is devoid of the philosophical element and renders it more akin to an emotion like joy or happiness.
When Aristotle uses pleasure in reference to tragedy, he is speaking in philosophical terms and not according to our current concept. Remembering that pleasure comes as the product of excellent activity that is fulfilled without hindrance or opposition, consider viewing a tragedy as an activity.
Viewing a Greek tragedy might well be considered an excellent activity as they were excellently crafted with high, elevated diction and objectives. Therefore, by definition, a Greek tragedy may produce pleasure. Further, a tragedy has qualities particular to it that are equally out of accord with comedy and monstrousness. The sensation duly associated with tragedy is to be that of the terrible. The appropriate product of pleasure would, as Aristotle states, then be pity and fear. In other words, if one feels pity for the characters, particularly the tragic hero, if one feels fear for the heroes ultimate end, one has engaged in an excellent activity that has produced pleasure proper to tragedy.
Those who employ spectacular means to create a sense not of the terrible but only of the monstrous, are strangers to the purpose of Tragedy; for we must not demand of Tragedy any and every kind of pleasure, but only that which is proper to it. And since the pleasure which the poet should afford is that which comes from pity and fear through imitation, it is evident that this quality must be impressed upon the incidents. (Poetics by Aristotle)