Why does the author find it hard to believe that his grandmother had been young and pretty once from chapter portrait of a lady?

In "Portrait of a Lady," the narrator finds it hard to believe that his grandmother had been young and pretty based on the way she is depicted in the portrait in the chapter. The portrait portrays a reserved, aged, and serious-looking woman, far removed from the youthful vibrancy and beauty that he associates with young and pretty people. This disconnect between the portrait and his understanding of youth and beauty creates a challenge for him in envisioning his grandmother as she once was.