You want to help people. It is common and natural to cite a desire to be help people as being one's motivation for pursuing a career in medicine; after all, that's exactly what doctors do. But, remember: actions speak louder than words. It is not enough to simply say you want to help others. You must be able to point to the specific activities you've engaged in that demonstrate your sincere desire to help. Moreover, explain why you have chosen medicine as the specific profession through which you will help people as opposed to all the other possible career paths through which you could accomplish the smae goal.
The dual nature of medicine. Many are drawn to medicine because it combines both science and humanism. If this is what motivated you to become a doctor, then be specific about what you mean by medicine's "dualistic" nature and how you've explored both of these sides. Demonstrate your passion for all the facets of medicine that you bring up.