As a predatory animal, we see the signs of weakness in our opponents as well as we see the signs of aggression. We recognize these aggressive patterns, and so may our opponent, if he isnt under the influence of some outside stimulus such as drugs or alchohol. However, if he is the antogonist, (if he starts the altercation) then he is likely not looking for signs of aggression but instead for signs of weakness which is why the preemptive strike works so many times.
"When I let Go of who I am, I become who I might be." Lao Tzu