Yup, I agree. She could be having a bad day, is tired, in a bad mood or you caught her off guard. It's hard to tell. Do you say hi to her first? It's possible she's just being nice and saying hi back, but for me it's because of one of the reasons stated before.
she mite be or desnt want to be bother. so if she is not smilin jjust say hello back. because it will at least make he smile in da inside.
hmm so. when I say hello and don't smile, I had a bad day. or is currently in a bad mood. it depends on the girl in question. if she's shy, then... she's shy. if she isnt, then she either is not happy or something like that.
but basically I always smile when I say hi. =) so yeah.