One factor explains a lot of what it means to be a good Jiu Jitsu training partner – your ability to match your opponent’s intensity.
This isn’t necessarily under conscious control, especially beginners who experience and respond to massive adrenaline dumps. This is why beginners are less trustworthy – they do not necessarily have control and pose greater threats to one’s longevity than a skilled black belt who, at any point, can injure you at will.
It begs the question, what other things in life do we not have control over due to naivety, ignorance, incompetence and inexperience? It makes one want to master everything to take control of one’s destiny, and exert will on the World.