Some critics believe that Lear in mad as the play begins. Why else would he divide up his kingdom? Why else would he wrest authority away, to his children? Why else would he devise a silly love contest in his kingdom's allotment? If not mad, these acts are certainly foolish.
His age doesn't help. Lear is old for that time. Senility and dementia are common for anyone that age, not to mention the added pressure of being King. Stress can ravage a body.
We certainly have a correlation here: a divided kingdom equals a divided mind. Any king who divides his kingdom is asking for problems, suffering, and unruliness. Worse, any father who divides his love between his children is setting himself and them up to fail. And any father or king who enacts a contest to determine awards is certainly asking for the worst in human behavior. Greed and ingratitude soon rear their ugly heads.
Casting away a child (Cordelia) and a trusted advisor (Kent) will cause a detachment from reality. Compound this with two selfish forces (Regan and Goneril) competing for land and Lear serves up a recipe for mental anguish. Critic Laurence Stern wrote:
Nobody, but he who has felt it, can conceive what a plaguing thing it is to have a man's mind torn asunder by two projects of equal strength, both obstinately pulling in a contrary direction at the same time.
So it is with Lear. Love and competition for love tear his mind into madness. He cannot reconcile the two. His madness is a retreat from reality, a kind of denial of reality. He cannot accept the fear that his daughters and subjects do no love him. He fears they may only want from him.
R. Moore, in his essay "Madness in King Lear," says:
Several things attribute to Lear's eventual madness. The Fool, initially, plays a large part in pointing out to the King his foolish mistakes. Even before the onset of Lear's madness, the Fool is anticipating it:
thou hast pared thy wit o'both sides, and left nothing i' the middle.
Lear's gradual realization of the disloyalty of his two elder daughters also leads him to anticipate his oncoming madness. Reproaching himself for his blindness, he says of himself, "Either his notion weakens, his discernings/ Are lethargied," (I.iv.236-37) and later, ". . . let thy folly in,/ And thy dear judgement out!" (I.iv.280-81) It is Lear's reaction to Goneril's refusal to house him together with his whole retinue that marks the first real premonition of his madness, and the Fool suggests that it is his lack of wisdom, which accompanied his old age, that will be the cause of it.