Proxemics research studies the influence of relative physical location of people on interaction and communication. The personal space is usually used for communication between family and friends. If a stranger or acquaintance comes closer than this range, they are perceived as invading one's personal space. This varies according to culture. North Americans have a larger personal zone than South Americans, for instance.
Eye contact mediates the effect of personal space.
When personal zones are violated, as in elevators, avoiding eye contact reduces the stress of the encounter. Eye contact has far broader effects. We look into each other's eyes for signals of what is going on inside. In our culture, eye contact signifies trust, confidence, and believability. But eye contact is meaningful in every culture.