No one factor led the United States into war with Great Britain in 1812. Instead, it was a combination of a number of grievances and threats that compelled the United States to declare war.
A rather intangible but overarching reason for the war was the assertion of American sovereignty in the face of British transgressions against American independence. A number of incidents in the early years of the nineteenth century made it clear that Great Britain was not going to respect the United States as a nation of equals. This was made clear in 1807 when the British warship HMS Leopard attacked and boarded the American frigate Chesapeake. This and other affronts were seen as an attack on American honor. Many felt that the United States still needed to show Great Britain that they were not a nation to be bullied. As such, the War of 1812 is sometimes referred to as the Second War for Independence.
Another reason that the United States went to war was over trade restrictions that Great Britain was imposing against France. Since Great Britain was at war with Napoleon, their navy was enforcing a trade embargo against their French enemy. This greatly threatened American economic interests, as France was one of the country's largest trading partners. The British navy was also forcing captured American sailors into their navy and merchant marine. This was a clear violation of the freedoms of Americans and angered many.
Other concerns, such as the British support of Native American raids in the Northwest Territory and the American desire to expand further west, increased tensions between the two nations. By 1812, tensions between the two countries were escalating even further, and the conflict became unavoidable. The United States declared war in June, the first time that the nation ever declared war on another country.