I don't know if this is strictly true. The problem is that as you get closer to someone (and learn what would really hurt them), you also tend to care for them more. This caring has the effect that by the time you know enough to truly devastate someone, you can't imagine doing it.
One story I read expressed it this way:
Everyone has three knots (
character flaws / weaknesses). To cast a
hex on someone you need to know two of there knots, so your spell can have power over them. However, if you know all three of their knots, you see them as the weak human that they are and either feel sorry for them or love them, making it impossible for you to hex them.
In the end it ends up being a tradeoff, which serves as protection for our
psyches. We try to manage our relationships so that power only goes to people who can be trusted with it. People get hurt when the system breaks down, either from
emotional overload (I'm really mad at you and don't care how much I hurt you.)or because someone has gotten close enough to hurt without learning how to care. (I have seen this happen to a couple of my friends; it is heartrending to have to watch.)