Stress
This job can make your brain hurt. Brain surgeons normally get around four weeks off per year. The rest of the time, they're opening up heads or conducting spinal surgery. It's not rare for a brain surgeon to work over sixty hours a week. Why so much? Well, this is a very specialized field of medicine. Sure, there are tons of doctors, but not very many people become brain surgeons...in part because they can't handle the stress.
Even after sixteen years of training there are innumerable stressors involved in neurosurgery. Some of this stress is due to long hours at the hospital, events in the operating room, publishing academic papers, dealing with insurance companies, handling any lawsuits, maintaining a life outside the office, and staying relevant in the ever-changing world of neurosurgery. But mostly, it's about trying not to kill people.