Learn from experience.
A girl only wants you when she realizes you don't want her.
If if it's meant to be, she'll come crawling back to you.
If she's just after you cause of the thrill of it and the high she gets out of having you, the minute you go back to her, she'll dump you again.

Seriously dude.....if she only wants you cause she can't have you, then she's not worth having in the first place.