Prior to the War of 1812, the British navy engaged in impressment, which meant that they would force American sailors to work on British ships.
One of the causes of the War of 1812 was because of the British navy forcing American sailors to work on their British ships. Great Britain would try to justify this by saying the sailors were British runaways and they needed to be forced to work on their ships. The United States felt like this was very embarrassing and disheartening to them, as they could not protect their own ships from seizures and kidnapping.
Americans were not happy with this and they eventually had enough. They declared war on Great Britain, because of impressment, but also because of the many other factors that made America fed up with Great Britain.