Lack of documentation, apparently. I had my passport and driver's license, but it wasn't good enough. I was asking to stay in Canada for I think it was 2 months over the summer to stay with my boyfriend, now he's my husband. No criminal record, always successfully entered Canada through another border port in the past, for even longer periods of time, with only my passport. I was sent back with a list of documents that I now have to bring with me every time I cross into Canada (as I am now flagged), things like proof of residence (3 of them), proof of employment or unemployment benefits etc, proof of funds for trip as well as proof of money in my bank accounts, proof of means of travel (like tickets), most recent income tax return. There's a few more but I forget what they are right now, can't find my paper that lists them.

Sometimes I get a visa that says if I don't return to the US within a certain amount of time (usually a week or two before my requested return date if I'm staying for a few months), I will be forcefully removed. Other times they will let me through without the visa, usually when I request only a week or two to visit. But I always have to go into the immigration office at the border and talk to someone and have them look over all my documents. This can take as little as 15 minutes or the longest stay we've had (apart from the time I was denied entry) was 2 hours. Sometimes they search our car while we're inside, other times they don't.

Now I'm working on immigrating to Canada so I can live here with my husband, so I have less recent US documents than before. Half of the time they tell me that I need more of a US paper trail; half of the time they tell me I need more of Canadian paper trail. It gets confusing. I never know if I'm going to be scolded or what.