Since this is a broad question, there will be many differences of opinion. That said, there are a number of events and movements in our history that define out attitude.
First, from our foundations, Christianity has played an important role. So, in some way, we are a Christian country. Some in America would not agree (because we live in a broad world), but it should be admitted that Christianity had an important role in shaping us. We can see this much more clearly when we leave the country. Countries with little Christian influence are very different.
Second, there is an attitude of independence. This is partially owing to the Revolutionary War, but also the pioneering spirit that developed this country into what it is today. It takes hard work to build a nation.
Third, race has always been an issue in our history. From slavery to the civil rights movement to what we are today, race has played an important role. Race relations have progressed, but there still is a lot of work to do. IAT test (you can look this up) show that people are still racist from a default point of view.
What makes Americans unique in their attitude is the sense of independence fostered from the beginnings of colonization and the distance from England. Colonists sought religious and economic freedom in the New World as well as the chance of a better life than was offered in Europe. Still a part of the British Empire during the eighteenth century, the British North American colonies uniquely represented providential opportunity with potential for personal autonomy.
Individual liberty was distinctly associated with the British colonies on the North American continent, and this concept derived from colonists having the freedom to trade without many enforced restrictions, as in Britain’s unofficial policy of “salutary neglect.” This laxness of enforcing the Navigation Acts in the colonies ensured prosperity, which benefitted the empire, and contributed greatly to colonial attitudes about self-government and their resentment of arbitrary rule, contributing to a unique colonial attitude about freedom and autonomy.