Asserting is to take a stand for oneself and one's position. To aggress (hardly ever used) or take aggression is to move to push someone from their position. As such aggression is regarded as more violent than mere assertion. Violence being generally distasteful is to be regarded as the poor choice. Though assertion can be overdone it is meant to be the persuasive, less overt form of persuasion. To each their own.

Asserting is to take a stand for oneself and one's position. To aggress (hardly ever used) or take aggression is to move to push someone from their position. As such aggression is regarded as more violent than mere assertion. Violence being generally distasteful is to be regarded as the poor choice. Though assertion can be overdone it is meant to be the persuasive, less overt form of aggression. To each their own.