Your medical school personal statement needs to sell. Every single word has to sell your reader and answer the question: "Is it worth my time to read one more word of this essay?" To be persuasive is to sell your reader on your next sentence, your next paragraph, your next idea of your medical school essay.
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.