Most people who have to reapply are not told what in their application was cause for rejection or lack of acceptance. Assume that ANYTHING in your app could have been the cause, and try your best to improve every aspect. If you think the essay is the best it could be, you should still try to rewrite it. I don't think it's horrible to use the same general essay idea or parts of the essay from last cycle, but really ask yourself if it's the best essay you could be sending.