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Few people would argue that assertiveness is a good thing. It is common for someone to tell themselves to be more assertive. We also hear advice from friends or mentors to speak up and be assertive.
If there is a general consensus that assertive communication is the ideal, then why is it so rare?
It most likely is because we do not have the skills to speak assertively.
The other two communication styles are passive and aggressive, which are opposite of each other.
A passive person allows others to take advantage of them; they give in and avoid conflicts.
An aggressive person takes advantage of others. They get their way and are rude, pushy, and demanding.
It is not surprising that a passive person and an aggressive person tend to find each other. They complement each other (not necessarily in a healthy way) like two puzzle pieces that fit together.
When a passive person finally decides that they have had enough and speaks up, their language, tone, and behavior becomes aggressive. This invariably backfires on them and the aggressive person pushes back, and things escalate.
The passive person does not have assertiveness skills. If they did, they would have been using them already. Their experience has only been passive or aggressive; those are the two styles they use.
They are afraid to speak up because when they tried in the past, it did not go well. It becomes more important for them to avoid conflicts and to have people like them — even when the consequence is the loss of self-respect.
Assertiveness allows for a win/win with both people feeling good about the interaction.