Personal essays appear to be part of today's trend in reading topics. Many magazines include a personal essay on a regular basis. Southern Living publishes Rick Bragg's thoughts on life on the last page of every issue. Writing magazines often use personal essays pertaining to the writing world while other specialty magazines highlight someone who has knowledge of their theme. The descriptive term personal is probably why these essays have become popular.
One of the reasons we write personal essays is just what the poster for today tells us. Write what should not be forgotten. The topic is often some small slice of life experienced by the author. So, why is a view of a pond while sitting on a bench or a twenty second happening on your way to work worthy of publication?
Why? Because you have found some universal truth in the experience. Huge? Not necessarily. It might be something very small but readers can relate to. That is what makes your essay interesting to a reader.
Of course, the personal essays that get published have more than that kernel of universal truth, even though it is still of prime importance. Besides that reason for readers to peruse your essay, it also needs to be written with sensory details, active rather than passive verbs, no cliches, and all the other things that make up what we simply call good writing. Some passion about your topic definitely helps, too.
I could write about something that I witnessed but never add anything about what I learned and it ends up being merely a report of a happening. If I include the lesson I received or some universal truth, the reader can agree or disagree but they might have gained a new awareness, too, or re-enforced something were aware of long ago.
You need not write a full paragraph about the universal truth or knowledge gained to conclude your essay. In fact, you're better off not expounding too greatly on it. A simple sentence will do just fine. You don't want to end up preaching. Simplicity is sensible. It doesn't need to be your final sentence either. This bit of knowledge can be woven into any part of your essay. Most often, however, it is going to be somewhere within the final few paragraphs.
There are a number of books about writing the personal essay. Google writing the personal essay and you'll find a treasure trove. Choose one or two to further your understanding of the personal essay technique and to explore the subject in depth.