Why the US should annex Canada: More land, more people, more GDP.
There is 1 main argument why Canadians are opposed and 1 main argument why American liberals are opposed.
The Canadian argument: Policy (namely Medicare for all).
If Canada joined the United States, then initially, every Canadian state would have Medicare for all and the rest of the United States wouldn't. Either Medicare for all is a net negative or it's a net positive. If the former is accurate, then the Canadian states would get rid of M4A voluntarily. If the ladder is accurate, then the rest of the North American United States copy the idea voluntarily. The process in favor of the superior idea winning gets expedited by competing ideas being in the same nation and we improve regardless; because it would be democracy in action and would allow for the battle place of ideas.
The US liberal argument: Policy (namely self-determination).
Self determination is objectively irrelevant; the liberals acknowledge this with which nation Crimea belongs too; they want Crimea to be with Ukraine even though most Crimeans want to join Russia. The liberals don't regard self determination with any consistent degree of relevancy and they should cease to pretend that they do.
I'm fine with Crimea being with Ukraine if Canada gets annexed by the United States.