Article 11 of the Articles of Confederation specifically says that Canada can join the US if it wants to. Was this mostly wishful thinking, or was there serious thought at the time that Canada wished to become a part of the US?
And I'm not aware of the US Constitution having any similar clauses. Is there any specific reason why it was included in the AoC but not in the Constitution? Was it kind of like the AoC was a draft (wrote it, implemented it, then realized what they should have done and did that with the Constitution)? Or did something change between the AoC and Constitution that made Canada less willing or less able to join the US? Considering the War of 1812, I'm assuming the US's desire to have Canada hadn't faded.
It was indeed, though more so before the American Civil War. Donald Warner's 1960 book, The Idea of Continental Union: Agitation for the Annexation of Canada to the United States, 1849-1893 is one of the more complete works. I'm more familiar with it in the context of British Columbian annexation, which ended as a serious topic about the time that word of confederation reached the colony from across the continent.
In early 1867, the Alaska Purchase treaty was signed, and this spurred a massive wave of discussion in British Columbia about the future of that colony. I wrote about this period extensively in an answer to a semi-related question.
In fall 1867, the talk of American annexation was serious enough that U.S. Army officers visited the colony on an advance mission to judge the likelihood of incorporating it alongside Alaska into the United States. Although the officers reported that two-thirds of the colony was in favor of annexation, the idea of Confederation was attractive enough to defuse the drive for annexation. In addition, the recovering economic fortunes of British Columbia (which at the time was in a deep recession) further undercut those who wished British Columbia to become just plain Columbia.