So Greg Mankiw wonders about why the Left makes a distinction between car insurance and health insurance and specifically when it comes to personal preferences. I think that he is baffled stems from the fact that he is not significantly surrounded by that many leftists as I am in Europe.
The thinking here goes along the lines of what is necessary for society, community and the nation in general. Although leftists like to eschew the label of "nationalism", in general however, a lot of their arguments rest on the notion of nationalism, which is quite interesting in its irony.
Mr. Mankiw is stupified by the distinction between a Prosche as a personal choice of car in comparison to children as a personal life-style choice. The distinction imo is that while cars are important, they don't have such a high level of necessity for a functioning society, children on the other hand have. For the continual survival of society and our species in general (and also for the welfare state) children are the basis and as such an important requirement. As such it should be a top priority for the state to do everything to increase the amount of children and their welfare. If it means subsidizing health care for their parents, then this is a to do for progressives. It is morally, ethically the right choice to do and on top of that a requirement derived from a general community spirit thinking.
I think this kind of thinking should not be as strange and foreign to Mankiw as he supposes in this post. After all the idea of civilization and society is also dear to the heart of conservativism.