Most Helpful Girl
Yeah, this really grinds my gears as well. If the guy (or girl in your case) has someone and is flirting back, they should probably state they are with someone so I'm not wasting my time.
I don't do this, because it irritates me. I'm guessing if a woman does this, she might just want or like the attention.3
Most Helpful Guy
In my opinion, girls like to get some attention. So, when they see a person interested in her, she feels really happy about it. She doesn't want to ruin it by exposing the fact that she already has a boyfriend.
Girls, they feel really happy when someone ask her out or tell her " I love you". It means so much for them, seriously.
Another option could be that she doesn't want to reveal her situation with her boyfriend to the public. By that, I mean she wants that the fact that she has a boyfriend remain a secret so no one will tease her on that because she doesn't like that. Something alike..1