Why would a same person always stare at you?

She may be. Or she could be admiring you. She could be jealous, envious, or even a little wistful. All women get those "stares" from one another, and it can mean a many different number of things. That's how we are (unfortunately). I say, don't worry about it and just remain causal and platonic. It doesn't have to mean anything at all.