Oddly enough, I knew the answer but couldn't formulate an answer on the spot. Now that I've thought about it, it's fairly simple and I'd be a fool not to share what I've rustled up in order to spread the knowledge.
It's actually quite simple. They want what they can't have. It's one of the most basic rules of attraction. It's how we grew up. As a child, if you have a toy, and put it down, as soon as another child grabs it, you suddenly want it back and fight over it. This deals with a similar concept. Women, and sometimes men as well, pursue their admirer only once they've seen how capable they are of maintaining a relationship.
I saw this on another forum recently . And i think you know the one i'm talking about. Though, woman usually tend to see competition in boys, i think woman like the fact that they have something to go with.. Something to keep them on the edge. And another girl is exactly that.
That's my opinion.
Now shaddup wimmins, and make me sum sammiches. /sarcasm