Friendliness
From the “smile” of course it all directs to their friendliness. It’s not even uncommon for them to chat with strangers while on public transportation, while waiting on line, while inside the elevator, and anywhere possible.
And don’t forget about the ultimate “How are you?” question, where you’ll expect answers like, “Good.”, “Not much.”, or “Fine.”. It’s only a common phrase or greeting that shouldn’t be taken literally, but for non-Americans, it could pose as a serious question and an honest answer might be given. That’s culture shock for both then.