That's not really the case. There were four main causes of the war of 1812:

1. The stopping and searching of American ships by the Royal Navy to seize and impress British deserters, especially the Chesapeake-Leopard affair of 1807 in which HMS Leopard fired on the USS Chesapeake after her captain refused to be searched.

2. The 1807 Orders in Council, which forbade neutral ships from trading with the French or any of Napoleon's allies, and meant that American ships were being stopped by the British.

3. British support for the Native Americans in the Old Northwest, especially the Shawnee under Tecumseh. The US wanted to clear out the Natives to make room for settlers, while the British wanted Tecumseh to maintain a buffer against American expansion.

4. American expansionism. This one's a bit more contentious but, it has been argued that the real cause for the war was to expand American territories into Canada. John Randolph, one of the opponents of the war at the time describe the war as being directed by "agricultural cupidity, not maritime rights" and many American politicians such as Henry Clay, Felix Grundy, and even Thomas Jefferson expressed a desire to wipe the British from "our continent".

None of these really constitute an attempt to reconquer the US by the British. Britain was certainly interfering with the US, and perhaps not treating her as an equal but Britain never tried to reconquer or reassert control over the US, prior to the war. Britain did try and conquer parts of the US during the war, particularly in New England, but that wasn't until 1814.

As to where the legend comes from, I can't say for sure, as I've not seen it as a widespread legend, but I'd imagine it is based on a misinterpretation of some of the War Hawks' rhetoric about the cause of the war. For example, John C. Calhoun called the war a "second struggle for liberty". He was referring to the British not backing down over the Orders in Council and the impressment of sailors, not because Britain was actually attempting to take back the US. This rhetoric, combined with the actual British attempts to annex parts of the US may be where what you were taught comes from, although that's just a hypothesis.

In school as a kid (in Britain), I was told the War of 1812 happened partially because the USA tried to conquer British Canada! Good to see we've both had unbiased educations!

In seriousness though, there was never any British intention to reconquer America after 1783. Th burning of Washington was even condemned by the British government as 'too far'. For an Ameican at the time however, you might perceive it very differently. Especially in the age before instant communication, rumours and speculation could escalate quickly. A savvy politician could even use such feelings to bolster support for the war, in much the same way fear of a German invasion of Britsin was used to drive recruitment in Britain during the early stages of the First World War, even though there was never any real threat of this.

With regards to the actual causes, KanBalamII has got the main ones. I think the invasion of Canada theory comes from the Articles of Confederation, which states that the province of Quebec could be added to the union later if the people there chose to do so. This did however not make it into the Constitution or any further Ameican policy, and the people of Canada remained loyal to the British.