Why the image formed in a plain mirror is laterally inverted?

The image formed by a plane mirror is always virtual (meaning that the light rays do not actually come from theimage), upright, and of the same shape and size as the object it is reflecting. ... However, the image is a laterally-inverted "mirror image" of the object

A plane mirror makes an image of objects in front of it; these images appear to be behind the plane in which the mirror lies. A straight line drawn from part of an object to the corresponding part of its image makes a right angle with, and is bisected by, the surface of the plane mirror. The image formed by a plane mirror is always virtual(meaning that the light rays do not actually come from the image), upright, and of the same shape and size as the object it is reflecting. A virtual image is a copy of an object formed at the location from which the light rays appear to come. However, the image is a laterally-inverted "mirror image" of the object. If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image.

Plane mirrors are the only type of mirror for which a real object always produces an image that is virtual, erect and of the same size as the object.