what is the use of filial ingratitude as the theme of king lear?

Filial ingratitude is a dominant theme in King Lear. It is a universal theme in the sense that it is common to find many sons and daughters who show much ingratitude and cruelty towards their parents....What made this play a tragedy was the evil children's "filial ingratitude," for the "blindness" of Lear and the Earl was so great that only through suffering from the "monster ingratitude" of Goneril, Regan, and Edmund did they learn to distinguish the good children from the evil ones. It was "filial ingratitude" which opened Lear's eyes to the "painful truth": he had disinherited his good daughter and had given power to his evil daughters.