One factor would be how America is a nation built on the premise of democracy and equality for all its citizens, yet established great security for civilians as well. Because of these strong values upheld by our nation, citizens felt it was our duty to help others who did not receive the same equality in their respective countries.
Despite the Farewell Address written by George Washington, where he said, "It is our true policy to steer clear of permanent alliances with any portion of the foreign world; so far, I mean, as we are now at liberty to do it; for let me not be understood as capable of patronizing infidelity to existing engagements."
We have come to Ronald Reagan's quote, "We cannot play innocents abroad in a world that is not innocent."