I based mine (at least this is how I started it and ended it) on an experience I had on my OB Anesthesia roation that really confirmed my decision to do anestheisa. IT was a little bit cheesy, but I guess that's what they are looking for. So basically my suggestion would be you could focus on a specific story about one of your anesthesia rotations, pateints, faculty, etc. and how it influenced you to peruse anesthesia. For me, I had trouble starting mine, but once I thought of that story, then it just flowed from there.
Good luck!