As he grew older and more experienced, successful, and confident as a playwright, Shakespeare learned he could express his own views and feelings through his characters. Cordelia here is telling the truth about human nature, evolution, and parent-child--especially father-daughter--relations. Little girls typically adore their fathers up to a certain age, but evolution has programmed them to turn their attentions and affections away to young and unrelated males. In modern life we see adolescent girls develop an interest in actors, rock stars, and others they used to call "those horrid boys!" Fathers, like Lear, continue to love their children as before, but they find themselves quarreling with their sons and have to realize that their daughters no longer consider them handsome, or wise, or funny, or anything else but rather quaint.
Cordelia is stating the simple fact that when girls reach adolescence they develop interests in males other than their fathers--although their fathers hopefully may serve as models of the kind of husbands the girls would like to have.
Good my lord,
You have begot me, bred me, loved me: I
Return those duties back as are right fit,
Obey you, love you, and most honor you.
Why have my sisters husbands, if they say
They love you all? Haply, when I shall wed,
That lord whose hand must take my plight shall carry
Half my love with him, half my care and duty:
Sure, I shall never marry like my sisters,
To love my father all.
Cordelia is already thinking about finding a husband, leaving home, leaving her father, having children and a home of her own. Lear is losing her, whether he likes it or not.
Goneril and Regan have long since seen through their dad and have completely broken away from him emotionally--but Lear still had hopes for Cordelia.
I loved her most, and thought to set my rest
On her kind nursery.
He is a selfish, inconsiderate old man. He counted on Cordelia because she was his youngest, still unmarried, still "his," still presumably attached to him and still under his influence. He wouldn't mind keeping her beside him until he died and she was too old to get married. One of the many things he has to learn through his coming ordeal is concern for other people.
Since Juliet was only thirteen in Romeo and Juliet, we might suppose that Cordelia is not much older. Lear is astonished by her apparent change, although he is only experiencing what most fathers will have to accept in their little girls when the time comes:
So young, and so untender?
Cordelia still seems as candid as a child. This seems to be our only way of explaining why she is so uncompromisingly honest. She speaks the truth because she doesn't know how to lie. Duplicity takes age and experience--but we all have to learn what the Fool tells Lear:
Truth's a dog must to kennel.
Cordelia's candor costs her one-third of a kingdom and eventually her life.