I've always wanted to be a doctor. If you use this approach, then there are two things you should be aware of. First, it is not enough to simply say you've always wanted to be a doctor and leave it at that. You need to show, not just state, your motivation. It's okay if you truly have always wanted to be a doctor since childhood, but admissions committees will want to if you've tested this decision over the years and approached it in a mature manner. Second, this approach has been done to death by generations of medical school applicants. To avoid the "here we go again" response from admissions officers, you should use an interesting, creative opening to catch their interest. Avoid beginning with a line "I've wanted to be a doctor since…" or your reader might not be willing to read past it.