I would like to see this moved to the Literature Discussion Forum so we can hear other peoples' interpretations of Aristotle's comment.
To me, Aristotle is adopting a somewhat cynical attitude, indicating his belief or theory that it is not possible to experience pleasure without experiencing tragedy. Further, I think he is saying that the greater the degree of pleasure, the more serious the related tragedy must be.
Personally, I don't think there has to be a direct correlation between positives and negatives in our lives, so I don't agree wholeheartedly with Aristotle's idea. On the other hand, our appreciation of the good things in life is heightened when we contrast them with hard times, so there is some basis for his viewpoint. Maybe the use of the words "pleasure" and "tragedy" adds extra layers of meaning, but, in my humble opinion, there is some truth to what he says.