Anytime there has been something of that description, Britain was always the motivator.

In Canada's position there are a few things discouraging it from picking a fight with its neighbor.

1.) Canada has always had a population dearth relative to the United States. Less people means less resources for industry, research, and, most importantly, the military. It has been perennially outnumbered.

2.) It does not control trade routes that are critical to US trade. With the exception of the Great Lakes, Canada has never had the geographic advantage to just sever a trade line.

2b.) Canada is a modern military, but like many other nations, does not have the Navy to match the US Navy. If they are going to project power over a coastline, you have to be able to go toe-to-toe with the people you are trying to restrict.

3.) The Canadian border, de-militarized as it is, is near impossible to hold without giving you plans. Either side of the border would have to deploy an unreasonable number of units along that border.

4.) The US and Canada share a very similar heritage. A product of British colonialism, a "frontier legacy", and a people who know each others culture well enough on friendly terms. For instance, the NHL has teams from both sides of the border. Our cultures are inter-twined enough that making the other seem like a true enemy would take a lot of propaganda work.

Those are all I can think of off the top of my head.

King Walnut has covered most of it in his post, but I'd like to add a little:

There was a war, the war of 1812(-16). The Americans were angry with several British policies, such as arming Native Americans and press-ganging suspected British-born Americans into the Navy to fight Napoleon. The reason Britain was arming Native Americans was that they were worried that western expansion of the USA would lead it to become powerful enough to be a threat to Britain, perhaps conquering Canada.

So, President Madison and his advisors decided to invade Canada, while Britain was engaged in the Napoleonic War and so unable to muster its full forces to defend Canada.

However, the war didn't go as well as expected. While the Americans did occupy a fair bit of territory, the war wasn't very popular at home, particularly in the north-east industrial areas, the areas most needed to support the nearby war. In addition, the British allied with a federation of allied native American tribes, led by the famous Tecumseh.

In 1815, the Napoleonic War ended. This meant Britain was, in theory, able to turn its full attention to America, and some Americans feared they could be reconquered. On the other hand, after a long, brutal and expensive war, the British frankly didn't have the appetite for another war in America (especially as their previous war in America hadn't gone that well).

So, the two countries signed an armistice. They promised to respect each other's borders, and not to sponsor forces opposed to each other (such as Native Americans). So, both sides got what they wanted: America no longer had Britain trying to stymy it's westward expansion, Britain felt security that its Canadian territories wouldn't be invaded by America.

After this, it was in neither Britain nor America's interests to fight a war. Trade was much more profitable. Now, resources could be seized by force... but why should either side want to attack a powerful, industrial country, one with legal standing in the world community, and a similar culture and religion? Why do that, when there was also this land to the west full of untapped resources, guarded by non-industrialised, low-population forces with essentially no international legal standing? For America, their military was too busy taking the West to want to invade icy Canada. For Britain, there was western Canada, but more importantly India and Africa to conquer (and the local Canadian forces lacked the population to attack America by themselves). By the time Canada became fully independent, we get to the era of the World Wars, where America and Britain ended up as allies. This was then followed by membership of NATO. In the modern era, cultural, trade and political ties are just too extensive for armed conflict, nor, as KingWalnut covered, is there any particular motivations to wage such a conflict.