You talking about from inside, or outside? If you mean fellow students and doctors, basically because it is a demanding job with increasing intrusion from outsiders telling us how we have to do our jobs, constant threat of being sued by those we are trying to help, and constant attacks on us and our profession from patients, family members of patients, the media, and the public in general.
If you mean why from outsiders, it is because physicians are seen as being "overpaid", we are the most highly educated individuals most patients ever actually see in person which can cause insecurity and hostility in some people, people feel entitled to get healthcare for free and so having to pay something for care, more specifically us expecting to be paid for our services, causes great hostility on the part of some patients.