Mona Lisa The History of the World's Most Famous Painting

What has made the Mona Lisa the most famous picture in the world? Why is it that, of all the 6,000 paintings in the Louvre, it is the only one to be exhibited in a special box, set in concrete and protected by two sheets of bulletproof glass? Why do thousands of visitors throng to see it every day, ignoring the masterpieces which surround it?