In conceptualizing a theme, you have to consider the main question, "What would you like to say with your essay?" For example, do you want the admissions committee to understand your choice of major and career in relation to your family background, or do you want to share your passion for your chosen field and your desire to achieve excellence in it? Whichever theme you come up with, this has to be unique to you as an applicant and should be related to your personal background, choice of major, and choice of career. The more unique a theme is, the more interesting the essay will be. In this sense, you can capitalize on personal facts you believe are noteworthy of exposition.
How to write a medical school personal statement: Motive _ Third is having a compelling motive. It's the underpinning of the whole essay. Unless you speak to a compelling motive, the default result from the reader's point of view is this student is writing a general medical school essay, but doesn't really know why. You've got to be willing to flaunt the motivation and be really willing to shout at the top of your lungs standing on a chair in the middle of a crowded auditorium. Be THAT committed to what you have to say. All of this sounds incredibly unrealistic and incredibly overdone, but once you put these concepts down on paper, the very act of reading mutes so much of the impact of language, that you've got to be willing to trust that you can be very committed and very over the top in how you articulate, and it will still carry off.