Doctors need to know a lot more than biology. You have to communicate with patients, find out health issues and concerns they may not be initially willing to tell you, be sensitive to issues in their background that may make particular health problems more likely or cause them to have communication issues ... you can be the best "human biologist" in the world, but if you're a conceited, overbearing jacka$$, your patients aren't going to want to mention that nagging twinge in their chest or that pain they experience during sex, and that could make all the difference in whether a problem gets diagnosed and treated. (For that matter, they won't want to continue being "your patients" at all.) Psychology, sociology, religious studies, communication, anthropology, etc., as undergraduate majors, can all give you the kind of cultural sensitivity and communication skills that may not be emphasized in med school but can make you a great doctor instead of an okay one. Unless you're planning to go into research or surgery, there's a lot more to medicine than biology.