Impressment was a major cause of the War. It's a term to describe the time British sailors were capturing American sailors and forcing them into the British navy. Another cause, dealing with Native Americans, was when America got angry at Britain because the British were supplying American Indians with guns to fight white settlers. The last major cause was dealing with the War Hawks. They advocated for war with Britain because at the time, Britain owned Canada and the War Hawks wanted it.
Tecumseh, a Native American, and his brother Tenskwatawa, banned together other Indians to fight against the white settlers for coming into their land. In the Battle of Tippecanoe, William Harrison led 1,000 men into battle and were ambushed by the Natives. The forces ended up returning to burn down Prophetstown, which was the center of the resistance. Angering the Natives, they deciding to join forces with Britain in the War of 1812.
War Hawks were congressmen who thought the only solution to American problems was war. James Madison was the president at the time, so that means they had to appeal to him. Their reasoning for wanting war was simply because they were sick and tired of Britain "controlling" the US. They also wanted Canada from Britain.