What are the sources of differences in ethics among people?
Differences in ethics arise for many reasons. Mostly, one's family, one's culture, one's education, and one's situation create these differences.
First, one's upbringing can create different ethical mindsets in people. For example, one might be raised in a family in which telling the truth is valued so highly that one can say something hurtful to another. Another family might value good manners over truth, and nurture the idea that telling little white lies is better than hurting someone's feelings. In a medical setting, a physician who is inclined the first way is going to tend to tell a patient a brutal truth, while a physician inclined the second way is going to tend to want to avoid telling a patient unpleasant truths. Of course, today, there is an ethical obligation to tell a patient everything, but how one is raised still has an effect on one's attitude about this.
A second source of ethical differences is one's culture. In some cultures, the needs of many are elevated over the needs of the individual, so ethical decisions are based on what will do the most people the most good. To a large degree, the United States promotes a highly individualist culture, where the needs of the individual are elevated over those of the society. In medicine, one example of how this gets sorted out is to examine what scheme is used to decide who gets an organ transplant. I would guess that there are some cultures in which a person who has children would be more likely to get a transplant than one who does not, because that is better for society than saving just one individual. Another example might be choosing between a heart surgeon and a ditch digger. Saving the heart surgeon is likely to be judged as more beneficial to society. (This is not to say that I endorse either of these, but to give you an idea of how a culture's priorities might influence medical choices.)
A third way in which differences arise in our ethical systems is through the educational process. Those people who study ethics in a formal, educational setting learn that there are different ways of approaching ethics, and thus, they can make a more informed choice about what ethical constructs they wish to live by. In the medical arena, each health care professional is taught and must subscribe to a formal set of ethics.
Finally, situations often dictate different ethical responses, and for all the early exposure, cultural influence, and education we receive, sometimes, our ethical responses to situations will vary considerably. Each of us has a unique personality in addition to all of the influences that have formed us, and many of us, when put to the test, are going to make a judgement based solely on the situation in which we find ourselves. A doctor on the battlefield, confronted with the dead and dying is going to make decisions based on his or her particular "here and now."
In spite of the fact that there are many differences amongst us from an ethical perspective, there are values that transcend upbringing, culture, education, and situation. For example, "Do no harm" is an ethical precept that everyone in health care subscribes to.