In Japan, the wearing of face masks is as popular as eating sushi for dinner. Everyone's at it.
The main function of the mask is to hide your face from other people so that they don't know what you're thinking. They are also used in bank raids.
In Japan, it is thought best not to give too much away regarding one's feelings, either through the use of larynx movements, or through the display of body gesticulations. That way, no one will be troubled by another person's negative words or actions, and social harmony will therefore be maintained. Nice.
The face is seen as a window into the soul, and if the owner of the face is unable to hide their emotions, then a mask will be used to do the job instead. If you were to grab the mask of an unsuspecting wearer and yank it politely from their face, you'd probably find they're trying to hide either a big cheesy grin or a picture of quiet sadness (or possibly some facial tics). On second thoughts, they'll probably have a really pissed off look, seeing that you've just yanked it from their face. So forget that idea.
So it can be said with a high degree of certainty that people wearing face masks are either a) hiding their emotions, or b) running from a bank.