Two main reasons:
1. She doesnt like you. Its easier to turn down a guy if you have a legitimate excuse. "Sorry I'm with someone" instead of "Sorry I really dont like you"
2. Shes playing herself up. Most girls have security problems. They always think that they need to be better than they already are. Girls dont just look at guys physically. They size them up on if they've ever been with someone or if someone else is interested in them. Therefore, they think guys think the same way. So if they say the have someone at the moment they think the guy will be even more interested in her because obviously someone else sees something in her! Its stupid but girls tend to over think things and get to caught up in their self image to realize that a guy doesnt care if someone else likes you too as long as he can have you.
simply tell the person you are in a relationship with that you are intrested in someone else and just be honest with them and tell them how you really feel.
Tell the guy you asked out that your sorry but you like someone else.
you cant get him to tell you get someone else
tell them to get over you and date someone else.
You should look for someone else to love.
Tell the person that you are crazy about them and tell them why, or else you'll regret not telling them
Well if it's not yours who's is it ! it has to be someone Else's.
"sorry, i like someone else" sincerely.
Ask her.
tell someone else
just tell that person how you fell about that he or she
Get over that person or tell it how you feel.