Manipulation of the kind you're talking about is going to involve flexibility of self - you have to be capable of being the person they would consider a friend, a lover, a confidant. This is significantly harder than it sounds, especially over long periods of time, and you run the very real risk of becoming the thing you only intended to pretend to be. This should be a matter of concern in serious matters - the necessity to be the person they need you to be means you are manipulated by them as a necessary element to manipulating them.

There's a reason countries tend to monitor the mental health of their spies pretty closely.