"Helping people" tends to be a bit too cliche and superficial to use as your interview answer, unless you can put more meat on the bones. In terms of volume, nurses probably see as many patients as physicians, and help takes many forms and spans many careers. So if you intent to go with "helping people", you need to delve a bit deeper than you have in the above post, I think.
To elaborate on this helping people them, I'd say something like "I enjoy helping people and collaborating with others. Medicine allows me to help people during some of the most frightening experiences in there lives. That's very meaningful to me. It also allows me to collaborate with nurses, technicians, physical therapists, nutritionists, and other doctors. Medicine is truly a team effort, and I work well in this setting."
When they ask "Well, why not become a nurse or a physician's assistant?"
I'd respond, "Those are great professions, and they do a lot of good, but I enjoy being the final point of care. If a patient needs help, I will be able to help them if there is nobody else around. I also enjoy the academic rigor that medical school entails."
This answer allows them to mark all the items in their mental checklist:
Isn't afraid of illness, death, and dying (check)
Recognizes that medicine is a team effort (check)
Has respect for all health care providers, not just doctors (check)
Enjoys the high academic intensity that medicine entails (check)
