Because that's not an easy thing to be aware of, particularly not when most societal norms train you to suppress showing more extreme manifestations of emotions.
Just try imagining spending a whole day visibly smiling happy.
People who know you will ask you what's the happy occurrence, strangers will think you either mock them or are simply crazy, that's how out of the ordinary visibly being happy is considered, which is just super sad.
Hmm... Interesting point. I'm going to try that today and see what happens :-) Umm... Of course I barely see anyone except my wife and people on the street (because I work at home), but I wonder how it will make me feel (other than having a tired mouth...)
