I hate to disagree with Grandmaster Yoda, but that title which quotes him is only partially true for anyone who deals with conflict on a regular basis.
Anger is a useful tool; it allows us to overcome inhibitions in times of stress, and to act. However if you are too angry you will become paralyzed with it. Being angry at the world all the time, makes us outsiders, makes us outcasts, and the tribe (society) tends to not trust us. Our words are lessened, even if our message is true.
Fear is a natural defense mechanism over what is bad for us. Phobias often are rooted in honest worries. For example a fear of spiders, is a fear of the alien, and of things which are toxic. But too much fear paralyses us too; so much that anger may not overcome it. It also overcomes rational thought. Rational thought coupled with a reasonable fear allows us act with expediency.
Aggression is natural. We can’t be meek and mild all the time; we must act with a certain level of aggression if we are going to get involved in conflict. As nice the idea of “the way of the peaceful warrior” is, it is not realistic in every instance. But too much aggression makes us a menace to the tribe. Some will say “what about Berserkers”. Well they could switch it on and off, and they were also feared by the tribe. When Cuchulain was taken by the warp spasm (the Irish equivalent to the berserk) he had to be plunged into vats of cold water repeatedly to snap him out of it, as he was a menace to the tribe.
It returns to the idea for warriors that life on this path is a balancing act.
So sorry Master Yoda I think you should have said “Too much anger, too much fear, too much aggression; the dark side of the psyche are they.” But what would I know, I’m not a 900 year old Jedi ;)