While aggressive behavior injures in order to win, assertive behavior focuses, not on winning as such, but on negotiating reasonable changes in the way both parties behave so as to equalize the balance of social power. The purpose of assertive speaking-up is usually to solve an interpersonal problem.
But assertiveness is not just expressing feelings, laying down the law to someone, and then walking away. In general, to solve problems you must do more than talk back or express feelings; you must be very clear about what you want to accomplish by asserting yourself. You must attend to your feelings, decide what you want, and then use some specific verbal skills to negotiate for the changes you want.
Assertive problem-solving involves the ability to plan, “sell,” and implement an agreeable contract between yourself and the other person without sounding like a nag, a dictator, or a preacher.
In other words, an assertive person can express feelings in a manner that is both personally satisfying and socially effective.
Sharon and Gordon Bower, Asserting Yourself