It's that sweet-yet-scolding note your roommate leaves about the one cup you left unwashed, or the report your colleague keeps "forgetting" to finish for you. Passive-aggression is frustrating to its targets, since it's not as easily identifiable—or unacceptable—as, say, socking someone in the jaw.
Demonstrating assertiveness means there's no question where you stand, no matter the topic. Cognitively, to be assertive implies a lack of anxious thoughts in light of stress. Behaviorally, assertiveness is all about asking for what you want in a manner that respects others.