Americans smile a lot.
It's not uncommon for an American to smile for no real reason when meeting eyes with a complete stranger, and that comes as a confusing surprise to many foreigners who don't have that same "smiling culture" in their home countries.
The Atlantic has a hypothesis about why Americans smile so much:
According to the video, it pretty much comes down to our history of immigration. The United States as we know it to day was created by immigrants of all kinds.
When you put a lot of people from very different backgrounds all into one place, non-verbal and symbolic forms of communication like gestures and facial expressions become a lot more important! Trust and cooperation are important in a growing society. If you want to tell someone unfamiliar that you're friendly or non-threatening, but it's hard to do that with words, you can still smile!
Reportedly, the United States isn't alone in this phenomenon. There seems to be a connection between countries with more diverse origins and using more non-verbal communication like smiling. Canada, for example, has a lot more smiling than China, which is far less diverse.
The more you know!