The Mona Lisa's eyes followed

A friend of mine mentioned that when he was a child and saw the Mona Lisa at the Louvre, her eyes seemed to follow him no matter where he moved. 

The phenomenon of "eyes following you" might be a simple physiological effect. 
However, I still find it curious that most paintings don’t appear to move or come alive. 
This is a bit puzzling because when we look at a painting, our gaze, perception, and senses are never completely still, even for a moment. 

I believe that a painting should either appear to move or possess the power or mechanism to make us forget that it’s not moving.