Agressiveness can be taught but takes some time to develop...it is 100% mental....
Not sure about that. It is mental but its much more deep rooted. Its part of the makeup of the player. Yes, it can be increased but its not natural for passive person to become aggressive, just because its required in the game. And the biggest issue for making someone aggressive, when they are not naturally, is the hesitation. Hesitation can play havoc by delaying timing of what happens and it can lead to more injuries as well as not being effective and being late. It only takes a fraction of a second but you can see it on the field.
Also, keep in mind that playing agressively takes a lot of energy. That one agressive scrum to win back the ball may sap a player's energy where they can't make a good offensive run for the next five minutes.
That's a problem. Need more conditioning and clearly, the players that can do both will be chosen over a player that cannot. This can be worked on and is straight forward, unlike aggressiveness.