When we use assertiveness, we are literally transforming the negative emotional energy of anger into positive energy, called, "well-being."  This form of self-esteem is defined as a feeling of comfort and satisfaction, as one's needs are met, a feeling of the maternal, or of being mothered. And so assertiveness is how mature adults, "mother themselves."

It is a constructive, "win/win" decision to make with one's anger, and takes mature character virtues of patience, generosity, and other mature ego defenses such as, perhaps, humor, anticipation and others, to execute on. One might say that it is the "higher-brained" use of anger.

When one does assertive actions, they are benefitting everyone, including themselves and their partner, as well as the environment and community, and the positive feedback they get from the environment teaches them something to bolster their mature decision-making, which is the amplification of the conscience and intuition stores that help us identify when we are doing right by others (ethics, conscience) and when we have chosen a beneficial environment in which to operate (shrewdness, intuition.)

This mode of anger expression is then one of the two principles behind the saying, "What doesn't kill us, makes us stronger." The stress, called, "hurt" that comes into our lives, the hunger or starvation for psychological resources, and  the aggression of others may be "alchemically" transformed into the self-esteem, called, "well-being" in this way, making us actually happier and more powerful for the experience, through assertiveness.

This decision to act maturely then, may also dispel our sadness or depression, defuse our aggression and that of others, and benefits our romance in the area of love, friendship and emotional attraction.