What is the psychology behind abuse of those we supposedly love?

Why would we ever want to hurt those we love? Jealousy may be a factor sometimes, but certainly not always. I think that even in the healthiest of relations there are times when we feel rage and are seized by an impulse to do harm, even if we do not give in to it. Why? And then there are the extreme cases of child and spouse abuse. How can such actions be done in the name of love?

In Being And Nothingness, Sartre has a long section on sado-masochism. He thinks that this is a normal part of all relationships. He says that the Other represents a threat to our freedom and sadism and masochism are ways of dealing with it, to either dominate or acquiesce. Can there be any truth to this? Surely there is more to a relationship than this, a desire to share and please and feel happiness in the accomplishments of those we love.