JQL it is indeed a simple sign of patriotism. Some like to broadcast it from their homes, others from their vehicles (or both). Corporate offices often have a flagpole near the front entrance and fly the flag as well.
The prisoner of war/missing in action (POW/MIA) flag is very commonly flown by veterans of foreign wars (and plenty of others as everyone has friends or family touched by war) showing sympathy for their lost or captured brothers and sisters and compassion to their families. It is a sign of respect to those who endured capture or gave the final sacrifice in service to their country. An acknowledgement that their sacrifice is honored, remembered, and respected. It's another sign of patriotism.
A thesis could probably be written as to why Americans fly the flag more so than in other parts of the world. I will comment that I think it is becoming less common than it once was. Which is unfortunate. National pride is good.
We're still a pretty young country compared to the rest of the world, so maybe flag flying is something the "new kids on the block" do and perhaps it will curtail as a few more centuries pass
This, of course, is but one man's opinion.
