The War of 1812 happened largely because the English had not really accepted the idea that the US was an independent country. The things that England did led Americans to want to go to war.
After the Revolutionary War, the United States became an independent country. However, the British did not seem completely willing to accept this. At the very least, they did not have much respect for American sovereignty and did not fear American retaliation for anything they did.
Because of this, the British did three main things that annoyed the Americans and helped lead to the war. First, they kept forts on American territory. These forts were in the area that was called the “Northwest” at that time. Second, the British used these forts to arm Indians and to provoke them into attacking American settlers. Finally, the British were not respecting the rights of American ships and sailors. They would stop American ships and would force some of the sailors on those ships to become part of the British navy. This was a practice known as impressment.
These were the main factors that caused the war to begin.