Why is Cordelia banished at the beginning of King Lear?

At the beginning of King Lear, Lear calls his daughters to him, deciding to give each a piece of his kingdom in proportion to her affection for him. Goneril and Regan flatter him with lies about their extreme love and devotion. Cordelia, his favorite and the joy of his life, is disgusted by her sister's lying flattery and states plainly that she loves him according to her duty as a daughter. That enrages him, and he banishes her.