The overriding principle that Regan and Goneril are motivated by is that the ends justify the means. They are more than willing to practice insincerity, deceit, and manipulation to achieve their ends. Since King Lear has a lifelong history of being unpredictable in his whims, it may seem to the reader (and perhaps to Regan and Goneril) that their accommodation to his demands is just a harmless and practical step in a game or a sort of family dance that is habitually played or stepped to. As events continue and as the women's true characters are revealed, the cruel and loveless qualities of their actions emerge from the shadows of whimsy:
then must we look to receive from his age,
not alone the imperfections of long-engraffed
condition, but therewithal the unruly waywardness
that infirm and choleric years bring with them.
Cordelia's overriding principle is sincerity and honesty. This however is tinged with the family stubbornness and whimsy for she risks everything--her happiness, her father's love, her father's happiness, her future--on a seemingly petulant insistence on speaking only spontaneously of emotions of her heart. In other words, if she loves her father above all else and with complete sincerity and ingenuousness, why not deign to honor him by truthfully saying so publicly to please him and as part of his scheme that is to give him great joy.
[Aside] What shall Cordelia do?
Love, and be silent.
Her eloquence with words could easily provide a way to say that while she prefers to speak of her love without coercion and without manipulated ends and without a spirit of false competition, she does love him above all else and will gladly say so for all the company to hear, not to win for herself riches and wealth but to honor her father and crown him with the love and praise she feels for him.
The principles of Lear's daughters differ in that the two daughters value cruel manipulation and deceit while the other one values sincerity, truth, spontaneity and honesty. However--the three have in common the principle that they will put their own principles first; each reaping their own kind of harm as a result thereof:
Re-enter KING LEAR, with CORDELIA dead in his arms; EDGAR, Captain, and others following
Howl, howl, howl, howl! O, you are men of stones:
Had I your tongues and eyes, I'ld use them so
That heaven's vault should crack. She's gone for ever!
I know when one is dead, and when one lives;
She's dead as earth.