The first point to keep in mind is that 150-200 words is very short, less than one double-spaced page of type, or about half a page of single spaced type. You don't have much more than a couple of paragraphs to develop a story with a beginning, a middle, and an end. Therefore, both your moral and your story will have to be very simple.
I would come up with a moral first, such as that seemingly bad occurrences can turn out to be the best thing that ever happened. Then, I would think up a story of just one or, at most, two scenes that make that point. In paragraph one, the protagonist (and I would limit the story to one character) could, for instance, get into a car crash that gave him brain damage that took away all his aggressive instincts. He could mourn that he had become gentle, laid back, and "feminine." Then in paragraph two, he could note how his relationships had improved since he became less competitive and describe how much happier he is. Then, you could end on the moral. When thinking about this, note that this answer is almost 200 words.