One of the main principles of medical ethics is something called distributive justice. What it means is that resources should be allocated as fairly as possible, and when resources are limited, they should be aimed towards bringing the greatest benefit for all, not some.
When students think about distributive justice, they often call to mind health care expenditures and waiting lists for specialists. They think of MRI's and other expensive tests. It is a little detached, but the truth is that distributive justice is a harsh reality in medical care.
The easiest example is mass casualty situations. The first rule of triage is treat who you can save. If you can't save them, then don't waste your resources. It's cold, but it's society's competing interest.
But you don't have to have a mass casualty to find limitations in resources. I have been in a lot of tough situations as a resident. I've had a full ICU and patients needing a unit bed. I've had simultaneous codes. I've had three patients all go into respiratory failure at the same time. There are a lot of tough choices to be made there.
Now, most non-medical folks cringe to hear things like that. 'We need more nurses' or more financial support, or more beds, or whatever else. They mean well, but they miss the point. There will always be limits, but the most precious resource isn't any of those things. As a physician, I am the limited resource. My attention is a valuable commodity, and how I allocate it can be the difference in who lives and who dies. That is a tough call to make, and if you want to be a doctor, it's a call you'll have to make.