The inner workings of the mind are a mystery we've hardly scratched the surface of understanding. His basic functions remained, but his demeanor and personality dramatically shifted. Let alone his survival. His case let us know that we don't know shit, relatively speaking.
They weren't happy, they became walking vegetables. They'd destroy the brain until they were manageable, not until they were "happy." Splitting the lobes became an actual treatment for epilepsy, though.