I am currently watching gayo daejuk kae . Red velvets performance just ended. While watching yeri's face i thought she can be sad on stage it shows but still she chose to give small smiles for her fans.  She's actually strong enough to show up and perform for her fans. You just cant force a person to be happy all the time. Sadness comes in unexpected times it can also stay for a few days some years. Yes thats still sadness.

Looking happy cant make up to what you feel. Even if you tell me that she is not being  profesional because of how she acts . For me at her age the way she acts right now is completely normal. If she wants to show to others how sad she is she can. Thats why idols would get lonely . You'd all blame them for the acts that you think doesn't suit them. 

What you say about others can affect them. 

Its easy to say how should a certain celebrity should act.  I just want to remind you they are not puppets or robots. They have feelings . You dont know whats running inside their minds. I know its your own opinion but still please think before you say anything. Be sensitive enough for others. Not everyone is as strong as you. That's all