Many people confuse assertiveness with aggressiveness.
Both aggressiveness and passivity are instinctive. Rather than being instinctual, assertiveness is a learned behavior. This learning takes some work, but is worth the effort. While both aggressiveness and passivity may still be appropriate in some circumstances, learning assertiveness gives a third, more flexible choice. This means that assertiveness is concerned with dealing with your own feelings about yourself and other people, as much as with the end result.