[disown]

Of course it's part propaganda, but it's also a result of history. Our borders were expanding for nearly 200 years and in the process we gained unimaginable amount of resources and land. For a long time, you could really get land ( cheaply or free ) and you could hope to improve your lot in life. Of course all that land and resources were a result of the mass extermination of the natives, but that was the reality. It's really fairly recently where we've pretty much stagnated. For most of US history, there was genuine reasons for the optimism of the american dream.