I agree with both the other replies. When I worked as a CNA, people always said "I could never do that", whether because of the difficulties, the messes, or the deaths of patients. I always took it to mean, "I would not have the strength to do that." Let's face it, nurses are a special breed, and that is something to be proud of. After all, I haven't heard of too many people who go into the profession for the money!!!!
I am not actually a nurse yet, I am in school completing my general education requirements before I can apply to nursing school, but I am, and have always been, a nurse at heart, and that is what really counts!!!!!
So take it as a complement, no matter how the person saying it THINKS they mean it, you deserve it that way.