Blind to Love, and Crazy for it.

Initially the story frames the theme of equal love, love that is felt and given in the same way, but by different people. On the other hand the story alludes to the impossibility of such a thing. Cordelia's love for her father is different than that of her sisters, and moreover Gloucester's love for Edmund and Edgar were not equal either. It's almost like Shakespeare wants to show the power of love, of commitment, and how the emotion cannot be containerized into one box or category. True love, is not equal love, and the lack of true love is the realm in which evil and deception exist.
Goneril and Regan love power more than they love their father. They love his position, his authority over others, and not the man. Cordelia, in her honesty, truly loves her father. Lear only wants to hear of the same type of love from all his daughters, and cannot comprehend the different love that Cordelia gives.
Similarly, Gloucester loves his sons, but this love is different. One is legitimate, the other not, and even though he tries to use lip service to describe his equal love for each, it is evident that this is not the case. It’s almost like he says, “I love you, BUT you’re illegitimate.” The but cancels the genuine proclamation of his love.
Lear willingly leaves his daughters, betrayed and bewildered. He realizes that it is not love that his daughters have proven with their words, but the lack thereof. And it is not until after he goes mad, that Lear understands the significance of Cordelia’s daughterly love.
Edmund betrays his father, because he has a realization of the difference of love his father has for him. He has his brother banished, and then his father blinded and humiliated and sent out as well. He has the same love for power as do Goneril and Regan. Again it is not until Gloucester is blind, homeless, and without his position, that he recognizes the meaning of his fatherly love for Edgar.
Edgar and Cordelia are the odd ones out, because of their honesty and commitment to genuine love for their fathers. Love is the binding factor in the story. All the wealth and power are frivolous in comparison to the honest love that exists between the remaining couples, Edgar and Gloucester, and Cordelia and Lear.