There's a phrase in medicine: zebra hunting. When you hear the beating of hooves, think horses, not zebras. What this means is that common things occur commonly, and so when presented with a set of symptoms, look for the common disease first. Even an uncommon presentation of a common problem is more likely than a common presentation of an uncommon problem. If you go looking for the unlikely instead of the obvious, you are hunting zebras.
And that's all well and good until it's your own friends and family. With patients, you can play the odds, calculate the risk, give people choices. With family, you cannot help but assume the worst, because you have seen the worst. It's never a simple cold: it's pneumonia. It's never a headache: it's an intracranial hemorrhage.
You have seen too much. You have been exposed to the worst case scenario on a daily basis. You have seen breast cancer fungate. You have seen ischemic bowel. You have seen everything wrong that can happen to the human body, and the slightest hint of such a fate sets off all sorts of alarms. It is too great a risk to hope for the best.
Having all the knowledge and trying to use it on loved ones, it is paralyzing. It is an exercise in fear. It is reading NTSB safety reports while on a plane flying through turbulence. You can only see the worst possible outcome. And that is part of what it is to be a doctor.