There are three time periods in American history that became so well-known and influential that I think a lot of people separate them in their minds . There's the American Civil War which happened in the mid-1800s and spawned all sorts of Civil War buffs. Then from the mid-1800s to the early 1900s we have what has been fondly termed "the Old West" (like that's when all the cowboy movies take place). And then the early 1900s marks the beginning of the American Industrial Era (also very important).
The Old West has since been romanticized by books and movies as this magical time where men where men and the town sheriff was a man of integrity. The "cowboy" became the American version of Arthor and his knights.
A facet that I find really interesting about the second half of the 19th century is that the Civil War basically produced the type of men that ended up settling the rest of the country. Many settlers and cowboys were ex-soldiers hardened by war. Broken families, desserters, soliders, formerly rich Southerners whose plantations were burned... all people desperate enough to risk the unknown.
Of course, not all were like this. But many were. And I think this is something to keep in mind when considering how American attitudes were formed. It was unique at the time to have another chance to rebuild one's life. With Europe mapped out and portioned, it would be nearly impossible for a farmer to just pick up everything and find new land to work.
But Americans did have that option. It's really no wonder that the idea of freedom was so idealized and this population of people was so fiercely protective of their claims.