You need it because you want to have understanding of how to write a reaction paper in order to make success of it. A good-written sample will provide you with the necessary information about the structure of this kind of paper, its content and the style as a whole. But this doesn't mean that there is no need for learning requirements after you saw a sample. Quite the opposite: you should have an exact imagination of the final look of your paper, so it is essential to learn the rules, formats and structure. The more so since not every sample reaction paper may be written the way your professor wants you to write. What is most likely, your professor wants you to do the next: to prove that you have read the given text; to prove that you totally understood it; and, finally, to demonstrate that you are able to use it in absolutely new and creative manner. Additionally, he certainly wants you to adhere to common accepted recommendations in regard to writing a good reaction paper. It has to be at least one full page, but not too broad and long, though; it cannot be any kind of summary; it has to be the evidence of the fact that you as a student understood the text and can handle a discussion about it; it has to include the reaction to the story itself, your impression and position; and, obviously, it has to be rather interesting to read.