Okay, I'll just pretend you to be someone from Mars to ignore the awkwardness of the following: Among humans, it is common place to make facial expression of openness, friendliness and many other words along those lines. That often means in practice: Smiling. It is supposed to ensure other parties of one's good intentions, to ease tensions, to reduce distrust or to simply seem more likable. For many people and while the exact extend of use may vary a lot, such "strategic" smiles are common place in every day live, so much, that their fakedness may hardly if at all enter the mind of those faking the smile or those recognizing it.
In the end, smiling is a sophisticated and multi-faceted tool to coordinate social interaction. Its raw core are simple emotions, its civilized use is one of making living together easier.
Now, I don't know what is up on Mars or what on Mars is considered a faked smile. But that pretty much is the situation down here.