I know that there were a variety of causes relating to the Napoleonic Wars and unresolved business from the War of Independence, although without consulting Wikipedia I can't remember any other than impressment of US sailors. I also remember hearing that some people in the US wanted to conquer Canada, but I'm not clear whether that was a widely accepted goal on the US.
So, what did cause the War of 1812? What was it about? Why did disagreements with Britain (which I'm sure were around for a while) result in war when they did? I'm sure this is something that people have written whole books about, but I'd appreciate an executive summary from someone knowledgeable.
If anyone's interested, the context of this question was someone bringing up James Madison as a "worst president" candidate because he let the White House get burned down. I pointed out that that might not be "failure", depending on what the war goals of the US were, but then I realized that I didn't really know what they were.
In addition to the issue of impressment at sea and the entangling issues of the Napoleonic Wars, another major source of friction between the US and Great Britain was the western frontier. In the region between the Appalachians and the Mississippi River conflicting claims between the new United States, the British Empire, and a variety of Indian groups resulted in decades of tension from the 1780s up to 1812.
Despite the cession of the region to the US in the Treaty of Paris, which ended the Revolution, the British continued to occupy the strategic fur trading posts of Detroit, Michilimackinac, and Fort Niagara. Between 1785 and 1795 the US waged war against the native inhabitants of the region, conflicts that pulled in opportunistic British imperialists- supporting America's enemies and improving relations with the tribes and confederacies could bolster the British fur trade (now based out of Montreal) and protect British North America's (Canada's) vulnerable southwest flank. During the war years presents and supplies flowed to the confederated tribes at the retained British posts, provoking hostility between the two powers.
Despite the Jay Treaty (1794), where the British finally agreed to vacate the 3 posts mentioned above, and the 1795 Treaty of Greenville, where much of present-day Ohio was ceded to the US by the northwest confederacy of Indians, tensions would soon return in the early 1800s. British and American tensions at sea caused the British in Canada to begin supporting Tecumseh, the Shawnee leader, who was building yet another confederacy to fight off American expansion further into the West. As before, the British were fighting something of a proxy war by 1811, a conflict that merged into the War of 1812.
Some sources (I've done at least one historiographical paper on this area):
Wright, J. Leitch, Jr. Britian and the American Frontier, 1783-1815. Athens, GA: University of Georgia Press, 1975.
Horsman, Reginald. “British Indian Policy in the Northwest, 1807-1812.” Misissippi Valley Historical Review 45, no. 1 (June 1958): 51-66.
Allen, Robert S. His Majesty’s Indian Allies: British Indian Policy in the Defence of Canada, 1774-1815. Toronto: Dundurn Press, 1992.
Stuart, Reginald. United States Expansionism and British North America, 1775-1871. Chapel Hill: University of North Carolina Press, 1988.