It's all a personality thing.
There are some guys who are just crazy, and they are balls out all the time. They also tend to be the team leaders, and hold their teammates accountable.
There are some guys who can be physical, but feed off the really crazy guys, or their coaches in order to get them fired up, because they can't maintain that intensity on their own.
And then there are some guys who are just pansies. They skated by on being faster and stronger most of their life and never had to or wanted to fight.
More broadly, as a team, I would say you can raise or lower that level by how you practice. If you're spending all of camp "wrapping up" or simulating tackling rather than actually popping, well then you're just not going to be as ready for that contact once the real stuff starts as someone who did.