If government is paying for everyone's health care, then it would seem to have an interest in promoting "healthy" lifestyles. This might sound innocuous, but it has troubling implications.
Consider, for example, the question of weight. Existing evidence seems to suggest that being overweight is bad for one's health and causes higher than average health care expenditure. So, government might want the health insurance plans it subsidizes to include incentives for exercise and weight loss.
Yet that approach runs headlong into a ban on pre-existing conditions, a crucial feature of Obamacare. This ban implies that overweight people get the same insurance, at the same price, as everyone else. This defeats an attempt to improve health by discouraging obesity and generates resentment from the non-obese who believe, accurately or not, that they are being forced to subsidize unhealthy behavior by others.
In a private, unregulated insurance market, competition between insurers would determine whether obesity actually predicts higher than average health expenditure. (Even if the obese are less healthy, their lifetime health expenditures might be near or below average because of shorter life expectancy.) Competition would determine whether provisions like required exercise regimes actually improve health. People who are overweight might face higher premiums, but they would bear the burden.
The same issue arises for an enormous range of behaviors: smoking, excessive drinking, downhill skiing, and so on. Government takeover of health insurance, implicitly or explicitly, takes a stand on all these issues. Government will not always get it right, no matter how well-intentioned, and competitive forces will not be allowed to correct the mistakes. In addition, imposition of a particular approach, with the implied cross-subsidies from the healthy to the unhealthy, constistutes one more way in which government intervention promotes an embittered, polarized society.