why is it when? (best answer 10 points)?

It's because you want them more now when they're with somebody else. It's kind of like when you play hard-to-get. When a girl plays hard to get you feel like you aren't good enough for her, which draws you in more and more to try to like her, similar to what is happening here.

You only want her because someone has interest in her. If she broke up with him tomorrow and asked you out, you would probably say no. Kinda like you like her cause you cant have her,

It might be jealousy kicking in. You see someone get what you could of had and now you want what they've got.

because you always want what you cant have. find someone else you do like. if you tell this chick you like her, she dumps her bf for you and you realize you dont like her, you kinda screwed yourselves.

Your jealous when you see her with someone else you get jealous.