Canadians are visa exempt when entering the U.S. As a result, most Canadians are deemed to be visitors for business or pleasure unless otherwise requested or limited by Customs and Border Protection (CBP), the Department of State, or the Department of Homeland Security. Of course, there are many visas available to Canadians, and these are necessary for gainful employment in the U.S., most courses of study in the U.S., and actually living in the U.S.
We get a lot of questions from Canadians who are confused about how much visiting they are allowed to do. There are two competing concepts causing this confusion - 1) period of stay allowed per entry, and 2) ties to Canada sufficient to present the case that one is not abandoning Canada. So, a Canadian can enter for up to 180 days on any given trip, but too many trips may lead to refusal or denial (the distinction between refusal and denial is another topic altogether) based on a presumption of intent to abandon Canada.
Under U.S. law, all persons seeking entry to the U.S. are presumed to be intending immigrants, and must prove by clear and convincing evidence that they do not intend to abandon their home countries. Because there are no constitutional protections afforded to aliens seeking admission, this is tantamount to 'presumed guilty until proven innocent.' Therefore, Canadians that spend too much time in the U.S., even if each trip is well under 180 days in length, may be turned away at the Port of Entry based upon a presumption that they do not have sufficient ties to Canada to prove an intent to return there.
Frequently, Canadians married or engaged to U.S. citizens are waiting for visas to be approved (K-1 fiance/fiancee, K-3 spousal, permanent residence, green card) and will run into this problem at the border or airport because they have already declared an intent to move to the U.S. upon approval of the visa. The best thing to do in this situation is to carry copies of all pending paperwork along with recent paystubs, proof of mortgage or lease in Canada, and any other proof of a definite event for which they must return or suffer significant inconvenience, such as medical appointments or tests at university, etc.