This is surprising to hear. From my perspective as accomplices scientist being a doctor strikes me as one of the few jobs where you are obviously and directly helping people, thus “making the world a better place” (as we CS folks sometimes like to think we do).
That probably depends heavily on the field and even then, the endless flow of new patients can make it seem hopeless. Sure you patched up two guys today, but there will be five more tomorrow and every day after that.
It's even worse if the patients in question don't just suffer from "unjust" illnesses that just happened by accident but are instead stubborn alcoholics or something like that. You can treat someone for the symptoms of liver failure, but you already know they'll be in again next week because they can't leave the bottle alone. And when this patient dies there will be hundreds more next week.
When my grandfather was in the hospital I got to talking with the patient in the next room, who was recovering from a quadruple heart bypass operation. He didn't like the hospital food, so he convinced his family to bring him a double cheeseburger and fries from the fast-food restaurant down the street. That's got to be a little disillusioning for doctors.
I have a few friends who are doctors. One is a primary care physician and he talks about the frustrations of patients refusing to take care of their health or adopt behavioral changes he recommends. Most just want a prescription so that they can continue their bad habits. The other said he went into surgery because he didn’t want to have to work with patients who won’t take care of themselves. However, he occasionally looses patients on the operating table which never gets easier. As an undergraduate I considered going into academia, and one professor cautioned that I need to be prepared to see the ugly side of academics and be willing to tolerate seeing something I love be sullied by inter-departmental politics, publish or perish, difficult students, etc. In short, I think all professions have their upsides and downsides. I think the answer is to care about your craft for your self respect. I think you have to do your best ultimately for yourself. We’re putting a lot of our time/life into our craft, so it seems to me we owe it to ourselves to make it mean something, regardless of the reality of coworkers, the need for a paycheck, etc. I will also admit that living up to such a challenge is a daily struggle, and I don’t always measure up to my own ideals. But I’m always trying.
For me it's losing the challenge/being disillusioned with the remaining goals