If you've ever spent any time around a veteran police officer or military personnel you will likely have been made aware of the concept of the personal bubble or personal space. This is because those with jobs where they are forced to regularly interact with those who wish them harm expand out that personal bubble, and moving inside that will put them on edge even if they like you.
This has gone so far for some people that it becomes difficult for them to sleep in the same bed with their wives, but even those of us who don't have those issues still have personal space that surrounds us and we become uncomfortable when someone moves inside it.
In addition to this personal bubble that surrounds you humans also have personal spaces. Locations such as their home, car and office which they feel protective of. Someone entering any of these without permission is almost certain to upset a person. What is it that causes this reaction though? Why do humans feel the instinctual need to keep people at a distance and what does this say about us as a species and as individuals?
One of the most basic assumptions is that personal space is a dominance issue, that similar to any other pack or tribal animal such as dogs or monkeys humans have a natural hierarchy and when someone challenges that hierarchy we have the same reaction that a dog might with the human equivalent of growling. This is supported by the understanding that men tend to have a larger personal bubble than women but with both the distance is typically that of a single arms length give or take a few inches.
As with any human interaction the effect is not simply instinctual but cultural and environmental as well. It has been well established through tests that those who live in areas of denser population have considerably smaller personal bubbles, so a person living in Japan is likely stand closer to you than someone from Mongolia. Culture also has an effect on this and many cultures have traditions that are designed in large part to break personal space. These traditions include hugging, handshakes, and kissing on the cheeks. All of these can in some way be seen as breaking the personal bubble as a form of greeting.
To expand out the theory of personal space more you can actually extrapolate 4 levels of personal space. Anything less than a foot and a half a way is consider inside your intimate space, anything up to 4 feet away is personal space, up to 12 feet is social space, and anything outside of that is public space.
The understanding of personal space can help you interact better with people, most notably those who have a large personal space. Be aware of how close you are to people and how they react. Simply taking a small step back can often reduce social discomfort considerably.