Serious look - default facial expression. Lol!
Sometimes I was already smiling in my mind but realized my default expression has'nt changed yet. No wonder the other person did not smile back.
Something similar has happened to me before haha! I had some friends that would think I was in a bad mood because of my facial expression, when, in reality I wasn't in a bad mood at all! I had to tell them that and from then on, they finally realized it!