Personally I find it really strange that we (the U.S.) aren't working every day towards having an open border with Canada (it doesn't seem to be just Canada that doesn't want it).

I guess you'd need limitations on health care and an awareness campaign about it (so that confused people didn't travel in an attempt to save money), but other than that, I think the only stopper is the slightly higher level of paranoia the U.S. shows towards foreign visitors.