It's also a form of ensuring themselves that they are superior to you. It's also an American myth that if you smile, somehow, magically, your day will be better because nobody likes speaking to a grouch. And if people will speak to you all the time because you're smiling, then you'll benefit in some way - perhaps meet a partner or make a sale or some other opening will come up.

Barbara Ehrenrich wrote a brilliant book on this called Bright-sided.

https://www.youtube.com/watch?v=KvwyhSeLZT8&t=103s

She showed that there was a direct correlation between the American economy sinking and everybody pretending all was well with their world.

The bottom line here is that if one refuses to accept negativity, one will not go out to solve the problems because one will not even dare to look at the problems.

There is also research that positive people die earlier because they are so positive that everything good is going to happen to them that they never see the dangers.

There's also a study somewhere that shows that depressed people contribute just as much to creativity and invention (if not more) than positive people.

For myself, it's one of the endless reasons, I no longer live in America. I would have liked to have met some sincere people. Unfortunately, I only met those online because, online, somehow, people were more prone to be themselves - on Google Plus, anyway.