How does filial ingratitude in King Lear lead to suffering and relate to his unjust decision?

William Shakespeare's King Lear is a play that centers around themes of filial ingratitude. At the beginning of the play, Lear decides to divide his kingdom among his three daughters. Rather than following the advice of his advisers, he asks each daughter to declare her love for him. The evil sisters Regan and Goneril flatter the king outrageously, conniving to inherit his estate, but Cordelia, the honest daughter speaks simply and plainly. Lear foolishly divides his estate between the two evil sisters, assuming that out of gratitude they will care for him in his old age.

Instead, he discovers that they are mean-spirited and lacking in filial piety, gradually stripping him of his escort and privileges and leaving him to starve. Thus his sufferings are due to lack of filial piety but also to his unjust decision to spurn his truly loving daughter Cordelia in favor of her dishonest, flattering sisters. 

The subplot concerning Gloucester and Edmund is similar, with Gloucester first favoring the evil Edmund, his illegitimate son, who turns on him causing him to suffer only to be rescued by his spurned son Edgar. 

The theme of filial ingratitude is best expressed in the famous exclamation:

"How sharper than a serpent's tooth it is to have a thankless child!"

Discuss the concept of filial ingratitude in King Lear.

There is a sense in which the whole play is dogged by Lear's incredible lack of understanding of his daughters. Cordelia's truthful response of "I love your Majesty / According to my bond, no more nor less" is noteworthy in comparison with the flowery, exaggerated and completely insincere declarations of love from Regan and Goneril, and yet Lear wants his pride flattered and the love his daughters have for him to be overt and public. The way in which he turns on the one true daughter who loves on him and surrenders himself into the hands of Regan and Goneril indicates his own foolishness.

However, having given away his authority to Regan and Goneril, he then goes on to see how his daughters only squander that power and bring his kingdom into a state of chaos. Appearance and reality have been confused by Lear, but it is only towards the end of the play that he is able to recognise how true Cordelia has been towards him, raising an army to save him from her sisters. Note Cordelia's words as she is finally reunited with her father in Act IV scene 7:

Restoration hang
Thy medicine on my lips, and let this kiss
Repair those violent harms that my two sisters
Have in thy reverence made!

Cordelia is a character who is explicitly linked therefore with reconciliation and restoration. Just as with her sisters, her true attitudes expose themselves during the course of the play and Lear comes to realise which of his daughters he can really trust.

Filial ingratitude is related to the dominant theme of the play, but in itself is not the play's central theme. Lear's relationship with each of his daughters serves to dramatize the concern and conflict that drives the play - the obligations and definitive nature of social roles. 

When Lear abdicates his throne and breaks up the kingdom, he is acting against nature. It is the nature of power and kingship to be the steward of power (that is, to keep it as long as possible). Lear's relationship with his daughters, uniform at the play's outset, is defined by his kingship within the family. 

He is the ruler, the authority, the "owner" of the land. His daughters and his subjects abide in his kingdom according to his wishes. When Lear announces that he wishes to take of the crown, he is attempting to reverse the natural order that existed (for himself, for his family, and for his country). 

This attempted reversal is evidence of a (literally) epic hubris, where his pride leads to an unveiling of the forces of nature that will either correct Lear's behavior or destroy him to restore the "natural order". 

Lear's downfall is the result of a tragic flaw in his character: his majestic sense of himself is not bounded by the norms of the natural order.

Lear's daughter's play a part in the corruption of the natural order, demonstrating a will to disrespect their father; to manipulate and deny him. ("Goneril treats Lear severely and appears quite monstrous.")

Yet, their part in Lear's tragedy is not descriptive of the larger tragedy he suffers, a crisis of identity, morality, and perspective wrought by forces outside of his family.

 ...the harm that Lear suffers at the hands of his older daughters, the reduction of his stature, is far surpassed by the cataclysmic destruction that the king calls down upon himself.