There are various theories about the so called "personal space".
Personal space is the region surrounding each person, or that area which a person considers his domain or territory. Often if entered by another being without this being desired, it makes them feel uncomfortable. The amount of space a being (person, plant, animal) needs falls into two categories, immediate individual physical space (determined by imagined boundaries), and the space an individual considers theirs to live in (often called habitat). These are dependent on many things, such as growth needs, habits, courtships, etc.

What distance is appropriate for a particular social situation depends on culture. It is also a matter of personal preference. People may feel uncomfortable if the distance is too large (cold) or too small (intrusive). It may be due to the limited available space, different cultural standards, physical intimacy, interpersonal relationships, or some form of rudeness. Permission is often expected if the intrusion is unexpected. Many customs are centered around just this particular issue.

Personal space is highly variable. Those who live in a densely populated environment tend to have smaller personal space requirements.
(wikipedia.org)

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