Some random thoughts:
One of my best friends is Canadian. From conversations with her, I think the American military is about 7 times larger than the Canadian military in terms of percentage of the national population and the military here has a fairly high mobility. So I imagine that impacts the overall culture.
I also know from talking to my international friends that, compared to other countries, Americans place a lot less value on personal connections. I am multicultural and many of my friends are international in part because I find the American definition of "friendship" to be too shallow. I have a foot in each camp, though, in that I am a military wife and we have moved a lot...well, compared to my childhood, where I lived in the same house from the time I was 3 until I got married and moved out. Compared to my husband's childhood, we have had a lot of stability. (His dad was also career military and he attended a different school every single year. The army doesn't do that so much anymore. It is too expensive.) I expect my special needs kids to live with me until my oldest is about 24 or 25. No biggie. It is what they need. But, hey, half of all American youth under age 25 live with their parents -- they just feel "defective" for doing so, most of the time. I think society is just complex and human brain doesn't FULLY mature until about age 25. Er, but that is a tangent. My point is that I am very close to my kids and it isn't some kind of problem. I don't step on their boundaries and they don't step on mine. For many Americans, such a close relationship would be "weird" and/or co-dependent and would cause serious difficulties for the individuals involved. It isn't a problem for us.
I also imagine that it is the legacy of how this country was founded. Many early settlers came over to escape persecution, etc. Excuse my ignorance because I don't really know how Canada was settled, but I think the way America began means that a lot of folks pick and move on to escape problems.
(And if Canada started much the same way, I may have just stuck my foot in my mouth. Oh well.) We still have a high percentage of immigrants compared to most countries, so those folks also picked up and moved on to make a new and better life for themselves and also are typically "scattered" from their extended family in a way that seems less common in most other countries, where the extended family can be a huge part of one's life.
So, it seems to me that it is just part of our culture and mindset and our immigration policies, military, and other things re-inforce it and keep it alive. Or maybe
lec9496 is right and it is more perception than reality.
