I have noticed people get a little frustrated or uncomfortable when a unknown or known person invade the personal space . How can one define Personal Space . Why do people get uncomfortable .
Personal Space may be seen as one's working area for carrying out autonomous actions and thoughts. When someone invades that space, you have decreased capacity to act autonomously. For example, when someone is physically too close, you have decreased freedom of locomotion and movement. When someone is being nosy or intellectually getting into your business, they are interfering with your ability to act by your own feelings and beliefs. The reason is that you begin to consider how that person might judge your actions and motivations. The key problem is interference with autonomy.
Put another way, invading someone's personal space interferes with that person's life, liberty, and pursuit of happiness by imposing external opinions or forces the individual does not desire in his or her life. Naturally social actions and interactions are meant to accommodate the feelings and desires of bystanders, but some actions are purely personal.
The situation is a bit different when we are talking about close friends or relatives. Presumably the reason here is that these external parties have already been screened for agreeability and likeability, so not only is there less likelihood of opposition, but there is also a personal desire to find agreement. Moreover, the less agreeable a person appears to be, the less you would want that person interfering with or even knowing your personal feelings, plans, and circumstances.