Some applicants may ramble on about themselves in a manner that may appear self-indulgent and not very appealing to the committee. Remember, this is an application essay, not an autobiography. Conversely, some applicants tend to say too little, perhaps hesitating to promote themselves too explicitly or not knowing what about themselves would be interesting to people whom they don't know. In such cases, perhaps focusing more on what you want to do than on what you have already done (let your record speak for itself) may help in getting beyond self-inhibition.
Most people don’t think of spiders when they bite into a tomato, but I do. Growing up in southern Ohio, I spent many summer afternoons picking baskets of tomatoes that would be canned or frozen and preserved for cold winter’s dinners. I loved the results of my labors, but I’ll never forget the sight of the enormous, black and white, scary-looking spiders that lived in the plants and created zigzag designs on their webs. In fact, those spiders, with their artistic web creations, inspired my interest in bugs and shaped my interest in science.