Most Helpful Girl
I agree with Wassup. It could really mean anything.
I personally always smile when I see someone else smiling. I'm usually thinking about how much I love when people smile. Really, I am.
Sometimes if someone looks friendly and is looking my way, I'll smile...because, well, I think they look friendly.
If I'm trying to cheer someone up, I'd give them an sympathetic smile in hopes of cheering them up somehow.
If you're having fun, you smile.. probably thinking about the fun you're having.. haha.
If you like someone, there's a good chance that they make you smile. :]
I guess it all truly depends on the situation and the two people involved.
I hope that helped somewhat. :]