Part of the answer is intellectual: how drugs are made, how they work in the body and how they affect other drugs, and the other part of the answer is personal: you get far more involved with people as a doctor than you do as a pharmacist, and that was one of the reasons I didn't and don't want to study medicine. Look at some of the stuff Dr Crippen has to deal with:
http://nhsblogdoc.blogspot.com/ I simply couldn't cope emotionally with some of the stuff he has to deal with. You can still build great relationships with your patients as a pharmacist, they just don't tend to go as deep as that of a doctor-patient relationship.