- Americans who are too self righteous for Manifest Destiny.
- Americans who hate Trump so much, anything he suggests or proposes must be a bad idea by default.
- Americans who hate Trump so much, they want him to fail even if it'll hurt the national interest, whatever that might be interpreted to be.
- Americans who are xenophobic, even though some Canadians (usually from Alberta or Saskatchewan) are arguably as or more American than themselves.
- Americans who are afraid territorial expansion may (further) overextend the US. This one is probably the most legitimate concern.
The list is actually reasonably lengthy, and also why while Trump, or more likely a successor may eventually succeed at annexing Canada, they'll more likely than not struggle, if not fail to secure the political mandate necessary for annexation by way of armed invasion.
In the unlikely event that the US invades Canada, the US may find itself both overextended and scrutinized elsewhere by countries worried about what American intentions will be for them.
Why bother invading such countries? Let them come to you!
War is pretty terrible. Try it if you want.
To be fair, if you want China to do well, you should aspire for China to win, whatever that is to you, without firing a single shot.
Of course they do! Have you not met any of them?!
You don't have to like them, but that isn't going to deter or alter their unrelenting belief in American Exceptionalism, however bizarre or illogical that might seem to you.
Many of them grew up equivocating greatness with America, and maybe that was or even still is true in certain contexts, and for better or worse, there isn't much that'll change their minds at the moment.
In case you missed it, there are plenty of supporters of the incoming president who are deeply sympathetic to the Confederate States of America, even if they're politically savvy enough to not vocalize it in public.