What was the reason for the war of 1812?

The American started the war because British ships had been attacking American ships and 'impressing' American sailors.

America had, prior to Trafalgar, been pushed around by both the British and French Empires and by 1812 they were still being pushed around by Britain and resented that. They wanted to show the Europeans that they weren't push overs and since Britain was now the only European country actively throwing their weight around they were the only targets.

The Perfect opportunity came in 1812 as Britain was heavily involved in the Peninsular War and were almost totally focused on Napoleon so the bulk of the British Empire forces would not be available to oppose the Americans.

They came up with a bunch of complaints against Britain and declared war, a couple of days later news reached them that Britain had agreed to meet all their demands before knowing the Americans had declared War but it was now too late to call the War off.

Now the focus came on whatever land America might be able to gain as a result of their war.

While there was a desire to annex British North America the War wasn't started over it. The War was started because the ten pound weakling that was the 1812 version of the USA wanted to show the heavyweight champion of the World, the British Empire, that it could and would stand up for itself.

It was a war of Pride more than anything else.

The Americans declared war in premise that they were resisting British raids on American ships to remove fugitive British sailors even though that practice had ended by agreement the year previous. The real reason was expanionist aims to conquer Canada and take her rich resources for the US. This aim failed utterly as every invasion was beaten back and the war ended by Treaty in 1815.

The British still resented the fact that the Americans won the Revolution... it was a last ditch effort to get their colony back.