Everything is energy. Here's an experiment. Without looking, pay attention to your right palm. How do you know your attention is on your palm and not your left ear, or the car parked outside? It's the feedback from the directed energy of your attention crossing your consciousness that makes you aware of the palm. Therefore, if someone is staring at you, their attention is directed at you. Where the attention goes, energy flows. You sense, become aware, or become conscious of the same energy as if you were paying attention to your palm.

We were all born with natural Psychic ability to see, hear, feel and know what is going in our environment. Energy travels to connect from point to point, and when the connection is made you feel it, and you know that one specific person is looking directly at you, and you look up, and you look right back into that person’s eyes, sealing in the connection of the one and same energy that is connecting the two of you. It is a spiritual connection of the universal oneness, and LOA that we each have within us.

Everyone can see what everyone else sees, like walking tv stations, all living things are transmitting every thought, image, decision, feeling, smell, sound and judgement. We all have decided to filter out other 'tv channels' and tune into only the subject matter that interests us, namely ourselves and our immediate environment. So whenever we come into view of someone's camera (their eyes) we, and everyone else around us, sees what is seen by that person. We can tell the exact direction of the picture based upon the picture seen. We know if it's coming from above, below, in front, behind, from near or far. We can tell how the viewer feels about us, what their decision is toward us and we automatically respond to them as if upon a telepathic video phone call, letting them know our response to their thoughts. Thats how thieves know who to rob, and who to steer clear of, and that's how you know, somehow, if you're approaching a safe area or a safe person, or not, and to decide your next move in advance.

Lets take for example, tennis. The balls travel at up to 100mph depending on the player. How does the opponent know where to run to meet that ball? They cant wait until it is struck to see the direction, for it moves like a lightning flash. They'd never make it. They must read the mind of their opponent so as to get into the right area BEFORE the arm is lifted and the ball is struck if they are to have any hope of getting across the court to meet it.

I hope this has added to the other wonderful explanations above and been of some help!

it is because you are aware that they are looking at you. you can be even more aware what do you sense from that?(feeling thoughts etc). experience and enjoy.