Because we literally depend on the USA for a number of things. In some ways, we are the USA's removed. In some cases that is terrible but in some ways we do benefit. For example: Thanks to the USA's outrageous warmongering due to their desire to keep their hegemony, we get to spend a lot less on our military. That money affords out current political system, socialist programs and social nets. The USA will help defend us because it is in their benefit to do so, both geopolitically and just geographically. We also need them on many economic levels, since they are our biggest trading partner.

Sadly, reality can be very multifaceted. Both for the good and the bad. But the old saying that when the USA sneezes Canada catches a cold, applies here.