Goneril and Regan are only extreme examples of human nature. Shakespeare was taking a common phenomenon, as he often did, and embellishing on it for dramatic purposes. Many children find their parents a burden and nuisance after they are fully grown and have their own families to raise and lives to lead. King Lear is an extreme example of an aging parent who becomes an annoyance. He even seems to realize this himself when he pretends to be speaking to his daughter Goneril:
Dear daughter, I confess that I am old;
Age is unnecessary: on my knees I beg That you'll vouchsafe me raiment, bed, and food.
This is a situation being enacted on a smaller scale in homes all across America. Married couples with families are confronted with the problem of caring for an aged father or mother. The parent may create conflict in their homes, but sending him or her to a nursing home can be ruinously expensive and may seem heartless. If the nursing home is close by, then the problem is only half resolved. And what if the old man or old woman refuses to go?
Here we have an extreme example of a common family problem. Lear not only wants to come and stay with his daughters—who have cut their own throats, so to speak, by publicly declaring their overwhelming, undying love for him—but he wants to bring a hundred drinking and hunting buddies, plus a Fool who is "all licensed" to insult everybody. It doesn't occur to Lear that he might be making a bit of a nuisance of himself. Older people are stereotyped as stubborn, impatient, inflexible, and demanding. Lear is all these things to the ultimate extreme.
In present day, the problems of grown children with aging parents are often much greater than they were in Shakespeare's day, simply because people live longer. In fact, they might come to seem immortal to their grown children. If a couple takes an aging parent into their home, they might be stuck with an increasingly difficult house guest for decades.