Paul Krugman

the reason anything aiming to cover a large fraction of the previously uninsured must either be single-payer or something very like the ACA — is the clearest thing I’ve seen in decades of policy discussion. Left to its own devices, a market economy won’t care for the sick unless they can pay for it; insurance can help up to a point, but insurance companies have no interest in covering people they suspect will get sick. Health care is an issue on which most people are natural Rawlsians: they can easily imagine themselves in the position of those who, through no fault of their own, experience expensive medical problems, and feel that society should protect people like themselves from such straits. You can tell insurance companies that they can’t discriminate based on medical history, but that means higher premiums for the healthy — and you also create an incentive to stay uninsured until or unless you get sick, which pushes premiums even higher.