When two animals or humans each believes they are dominant over the other, and one enters the personal space of the other; that is grounds for an immediate confrontation to determine who really is the dominant one.
This is done mostly unconsciously, and the body may even go into an attack position to scare off the invader.
More recently, researchers at UCLA have found that neurons in areas of the brain responsible for movement, vision, and memory lit up when a person performed an action, but in tantalizing similarity, they also lit up when a person watched that action being performed.
Open for Business: If a man's chest and torso are unobstructed by his arms, he's feeling open, both physically and mentally, to the woman he's looking at or interacting with. Buff Behavior: When a man sucks in his stomach, puffs out his chest, stands as tall as his stature will allow, and flexes his muscles, he's doing his utmost to appear as robust and buff as possible.
This is his attempt to seem fit for doing business with the woman he considers to be a hot item.
For 30 more signals that he's into you, check out The Body Language of Dating: Read His Signals, Send Your Own, and Get the Guy.
Everyone has a space around their body that is held sacred as their own territory.
This is called "personal space" and it is characterized by a zone or "bubble" that varies with individuals and circumstances.