Why do some girls stare and smile?

there's this girl that was in the same table with me and stared and smiled at me. i didn't smile bec. it seemed weird to stare and smile and not say anything when your right there in same table. anyways i was in th elounge sitting down (we live in the same building) and she's on her way out of the building and i see her smiling but she isn't looking at me.