The main difference is that health care is *insurance*. Insurance only works when more people participate than actually need it (or take less money out than others). Can you imagine being in a skiing accident and needing $100k in medical bills and being the only participant in your medical policy. You'd have to pay $100k + overhead.
If only sick people were a part of the insurance then the costs for that care would be averaged out to only be paid by the sick people.
The question is, should healthy people be forced to subsidize the care of those that are chronically ill?
Nah, healthy people won't need coverage for skiing accidents or acute leukemia. And if they did then they'd be winners since they only participate in the healthy people's insurance pool which would have lower premiums than the sick people's pool.