This is because that when people squint their eyes their eyes adjust by themselves to overcome the optical defect in their vision. When people are seeing things in a very far distance, they may not see the things very clearly because the light given out by the things in the far distance can not form a distinct image on the retina, which is partly due to the strong illumiantion around. But if the people squint their eyes, the eye will be like a pair of pinhole glasses to avoid the surrounding mussy lights entering into the eyes, so that the central lights from the distant things can directly form a distinct image on the retina. In spite of the fact that squinting eyes can make people see clearly, it is not a good habit to usually squint eyes because it will transform the shape of eyes as well as worsening the eyesight.
