It means that they don't trust anyone but the person that changes them. Like, for girls turning a boy, they fall deeper in love with the person then they already were, since their emotions higher. It's like, a pledge of loyalty.
Rayah
I look at him when he finishes and hesitate, "I'm going to kill you know, okay? You'll come back to life though."
All you need is faith, trust, and a little bit of pixie-dust!
He can't get better unless the changer compells them to. Though, they have to practice over...and over...and over, for years until they can finally control their compelling. So, Jace's emotion's with heighten and he'll practically throw everyone away just for her. You know that right?
Rayah
I watch, getting a bloodbag ready for him.
All you need is faith, trust, and a little bit of pixie-dust!
I smile fully and sit up slightly, everything still slightly sore. With a moment's hesitation, I sink my newfound fangs into the bag and close my eyes.
I'm never what I like I'm double sided And I just can't hide I kind of like it When I make you cry 'Cause I'm twisted up, twisted up Inside
May you never steal, lie, or cheat. But if you must steal, then steal away my sorrows. And if you must lie, then lie with me all the nights of your life. And if you must cheat, then please, cheat death. — An Unknown Bride, Leap Year