Submission and aggression are the two extremes that would bring more bad than good.
If assertiveness is the fulcrum in the center of the sea saw, submission and aggression are the two opposite extremes.
Submission is a behavior that
1) Hide your feelings
2) Don't know how to say no
3) Apologizing even when it is not your fault
4) Let others take advantage of you
5) Say what others want to hear
Aggression usually means all the opposite of above, with the hint that other people's feelings are ignored.
Both of them are not good behaviors because they would prevent you from getting what you want in your career.
Assertiveness is the mid-point where you
1) Show respect for other people
2) Be honest
3) Express your feelings
4) Learn to stand your ground
5) Be able to say no