Control of others gives them a sense of importance. By being assertive, on the other hand, your aim is not to feed your vanity or your ego. Ideally, assertive people are motivated not by hate, but by love. When they fight, it's only to protect ideals and values that they strongly believe in. When they try to protect their dignity as a human being, not to mention their physical and mental health, they are simultaneously protecting their ability to give to those whom they love.
Aggressive people are destructive.
Assertive people are constructive. A major reason is fear. It seems to me that this fear often intensifies when the other party is someone very close to you. You might be afraid of being condemned - as incompetent, stupid,lazy, inconsiderate, or whatever. When someone makes an unreasonable demand on you, you don't want to be thought of as selfish.