Your unconscious is constantly processing the stimuli around you.
Take a second, how does your skin feel right now? Are the hairs on the side near a person more tense? Are they super relaxed next to your BFF?
Your body is picking up how you feel towards this person and reacting before your mind does. Your body is always the first to react. This is your "gut feeling". Because of this, you might make decisions you have no idea why you did. Your body knew there was danger or risk and reacted.