It is still a sign of happiness there. Only these people do not believe that one can be happy all the time so constant smiling comes off as insincere. There is

It is still a sign of happiness there. Only these people do not believe that one can be happy all the time so constant smiling comes off as insincere. There is a Russian saying "Смех без причины - признак дурочины" — smiling without a reason is a sign of stupidity.