The organizational structure you put people in has a huge bearing on how they perform.
If a person is in a small town, they'll act differently than a large city.
It's not that they'll consciously choose different options, per se - instead, they'll naturally have different stimuli.
Add in different constraints and incentives, and you get very different behavior.
Why did the Romans conquer so much?
Largely because they were incredibly aggressive. The Roman Republic / Principate / Empire was basically almost always at war.
But, I seriously doubt that individuals Romans were so much more aggressive than their neighbors in Antiquity.
Rather, Roman Consuls -- commanders -- were elected for a term of one year.
They were expected to make their name for themselves in that year, so they immediately set out on a fast pace of battles.
What happened 14 months forwards was far less relevant than the next 12 months. A lot of traditional "conservation of energy" type principles would be thrown off.
And, of course, there was constant honeymoon periods -- less cynicism and exhaustion. A man elected to consul would be at peak excitement, energy level, and feeling triumphant going into battle, which would have a huge effect on moral factors.
The structure, in this case, determined the aggression. By appreciating glory and having short terms of election for military commanders, constant warfare was almost guaranteed.