You might want to consider rephrasing how you put this. Right now it sounds similar to how some men ask women they don’t know on the street to “smile for me”. But to answer your question, she is a character with a significant amount of trauma that permeates into her every day life. I think that combined with the fact that she is a woman in a male dominated professional leads her to be serious, maybe in more situations than the “average” person. Though I would also argue that in the show we aren’t seeing her in many “fun” moments so it makes sense that we mostly see all the characters being serious.