For what reasons would a girl smile at a boy?

Smiling from the opposite sex could be a sign of attention or fancying but don't be too self confident to approach because you cannot count on it. Walk past through or smile back and see the reaction of each. Maybe ask her any question or open a conversation which seems really normal and see the way she reacts.

It means she likes to smile. It does not mean she is mad for you and has to have you everywhere at the drop of a hat. But, the next time she smiles at you, go ahead a smile back. Then do a little dance, like the Moonwalk. If she laughs, proceed to introduce yourself.

Smiling is definitely one of the best beauty remedies. If you have a good sense of humor and a good approach to life, that's beautiful.