Self-deception is really the catalyst behind the downfall of these two men. Ironically, it is their "blindness", both metaphorical and for Gloucester literal, that allows them to regain their clarity.
Gloucester allows himself to be deceived by the bastard son Edmund who is a product of Gloucester's carnal desires. His poor judgment leads to his eyes being gouged out. When he is turned out on the heath, he allows himself to be led by a "madman" who he doesn't even realize is his own son.
Lear, too, is deceived but his deception is rooted in his arrogance and his need to have his children tell him what he wants to hear. When Cordelia speaks the truth, his ego is bruised and he lashes out because at this point he is more king than father. Kent even warns Lear to "see better."