The first reason why the United States fought Britain in the War of 1812 was to end the practice of impressment against American sailors. The United States traded with Britain and France during the War of 1812, going against blockades in both countries. Britain impressed American sailors onto its ships in order to maintain its own merchant marine. The United States claimed that it was free to trade with whomever it wished, and that British impressment was an insult to American sovereignty.
The second reason was the belief that Britain incited Native American attacks against American settlements in the Great Lakes region. British forts still occupied the area despite the ruling of the 1783 Treaty of Paris and many tribes traded at these forts. The main source of settler/indigenous animosity was white encroachment on Native lands.
Finally, there was a small group in Congress called the "War Hawks" who thought that the United States should annex Canada. It was their belief that the Canadians (who were British colonial subjects at the time) would welcome the Americans as liberators. They thought the period during the Napoleonic War would be perfect for this annexation, as Britain was not paying attention to its Canadian provinces. The Canadians fought off the Americans in their poorly managed invasion, and the British army turned out in force to repel the invading Americans.