Smiling can usually get another smile as a reaction.
It's practically a reflex to human kind to smile at someone when they smile at you.
She smiled at most everybody all day, everyday.
Her smile was not genuine no matter how it seemed.
It was the smile of a soldier after war. One where you know they've seen things they can never forget, but still choose to go on with life and make everyone else happy.
To get the positive reaction from them.