Why is Celestine always smiling?

I think she finds most of them, to varying degrees, pathetic and somewhat amusing. Plus, they are the moments when the servant/master relationship has broken down, and the balance of power shifts, often favouring her. She gains control. She smiles.

She didn't smile much at Joseph, because he was a threat. She only did when she tried to entrap him. And for that matter, after she was married and had entrapped the Captain, she became somewhat like her ex Madame - cold and distant, and unsmiling. So maybe the smile was more about the power games... once she had won, the smiling stopped...

All in all, there's much not difference between the working classes, and the wealthy in this film. Give half a chance, they all behave much alike. Its quite depressing, really.

to survive. all the men were terrible.