When I lead people through a body relaxation, I tend to spend a lot of time on the face. I am not sure why I started to do this, I just found myself talking more and more about softening and relaxing in the face. Perhaps it is because this is where we often see tension. It is the most public area of our bodies, where we are on display to the world.
We have a lot of control over our faces. We try to present a certain face to the world, and we are careful in case our facial expression gives us away. It is not only poker players who learn to control their faces.
However, it is hard to get the face to do what you want and to look natural at the same time. This is one reason why acting is a lot more difficult than it appears. It is hard make a smile convincing if the associated emotions are not there. So we might find we go through the day holding our face; we grin and bear it, as the expression goes.