Every cop has a reputation out on the street. Smart cops learn that perception plays a big part in how people react and interact with us. How do they view us? Are we too professional and aloof/not an easy person to talk to? Are we talkative and friendly while still staying in control?
Smart cops realize we are playing a psychological mind game when we deal with the bad guys. Just as animals can sense a weakness or fear, bad guys can also sense those traits in us. This makes some bad guys take a chance and resist arrest instead of complying with our commands.