Scarcity

Many on the left do not understand economics in the slightest.

Economics is the study of finite goods that have alternative uses. (Would someone mind checking to see if I got the first line of Basic Economics by Thomas Sowell correct.)

A quote from the Austrian Business Cycle:

"Man is confronted with a world of physical scarcity. That is, not all of our wants and needs, which are practically limitless, can be met. Outside of the Garden of Eden, we must produce in order to consume, and this means that we must combine our labor with whatever nature-given resources are available to us. As inherently rational beings, men have come to recognize many ways of solving this problem, such as peaceful cooperation under the division of labor leading to enhanced productivity, and private property rights permitting economic calculation so that different courses of action can be meaningfully compared."

Those on the left want all sorts of good things for everyone.  "Free" healthcare, "free" education, "free" art, etc.

What they do not understand is that someone needs to provide those things for them.  If a doctor is forced to work against his will, in order to provide "free" healthcare to someone, then that is slavery.