Avoid Cliches. There are a few standard reasons why people become doctors, and so it is very easy to fall into cliched expressions and ideas. Though it's fine to express a commonly_held notion, try to find a personal way to express it. For example, rather than write "I want to be a doctor because I want to help people," discuss the ways that you have shown your generosity and kindheartedness in the past; bring specific examples__such as your volunteer experiences__so as to avoid a direct (and possibly cliched) declaration of your point. Sometimes cliches are unavoidable; just make sure your essay isn't full of them.
But you've got to sound: Like you care a lot, and like you're completely devoted to it. An example: "I am completely devoted to medicine for the rest of my life. It's everything for me because it's the only thing that's going to allow me to express who I am personally and professionally." High_impact language has this very strong absolute, emotionally rich, concrete side to it where as a reader I am completely engrossed, engaged, enmeshed. I buy it. I believe you. Some of the best places to look for this kind of writing is where there is someone who is making a very impassioned speech. One of the best places that I know is in the most powerful, compelling, moving speeches of our time. Like Martin Luther King's "I Have A Dream," or speeches by Gandhi, or speeches by John F. Kennedy, or speeches by Barack Obama. Speeches that really elevate you and make you feel that there's hope for you, hope for us and that there's something bigger to strive for.