What the hell is wrong with her?!?

I know I'm going to regret asking this, but seriously? I have this friend, she's my best friend, and I love her as if she were my sister, perhaps moreso. Yet every time, she gets infatuated with a guy, she complains about his moral and character defects, sees him for the player that he is, yet starts dating him and complains when he ends up true to type and betrays her asking herself "Why didn't I see this coming?". She obviously sees this from the start, yet goes against her better judgment and gets hurt time and again. And each time it breaks my own heart to see her like this. What can I do or say to get her to wake up and start thinking smarter already?