Hold me, hold me, never let me go;

Why do people continue to date people who are clearly not good for them?
I really just can't understand why people do things like that.

My friend Cheyenne is dating this boy named John, who she started going out with a year ago.
At first he was nice but then he started getting kind of violent and annoying.
But I put up with him because Chey is my friend, and I didn't want to upset her.
Now he controls her.
He won't let her do anything without his permission.
And he plays mind games with her like telling her he cheated on her and breaking up with her constantly.
I'm sick of it, but every time you try to tell her how you feel about John and her, she gets mad.
So I gave up.
All he does is treat her like crap, but she can't seem to stop "loving" him.

Then Amanda met this boy named Sean at the mall and they started going out.
I thought he was good for her a first, but then Lindy told me that he had been cheating on Amanda.
But when we tried to tell her, she didn't believe us.
So then last weekend at the mall they broke up, and she was saying all this stuff about how she was happy hey broke up, and that she wouldn't go back to him, but she is dating him AGAIN!

I just don't understand.
There are plenty of guys in the world, but some girls just seem to get stuck up on one.