Think about this. What kind of person would willingly cut him/herself off from 90% of the economy and people of North America, for the privilege of paying higher taxes, and maybe getting some "free" health-care in return? I'm sure some people would. When someone has limited resources, is in poor health, or is past his most productive years, the temptation to kick back and let someone else look after you may be strong. However, for everyone else, the restrictions imposed on Canada's economy by the US border, increasingly become a losing proposition. The border starts to represent a line which prevents you from accessing all the opportunities of the continent, and keeps you poorer than you otherwise need to be.
Most Canadians who find themselves in the latter situation, feel they have no choice in the matter. Rather than make themselves unhappy over it, they justify their situation with the "at least we have free health-care" argument. That's all it is. A mental justification for what they know in their hearts is not an optimal situation for them. If they truly had the choice of getting a green card or voting for a UNA, I suspect the feelings they would display then would be quite different.