From The Atlantic [theatlantic.com]:
It is a good question, but I was a little surprised to see it as the title of a research paper in a medical journal: “How Happy Is Too Happy?”
Yet there it was in a publication [tandfonline.com] from 2012. The article was grappling with the issue of how we should deal with the possibility of manipulating people’s moods and feelings of happiness through brain stimulation. If you have direct access to the reward system and can turn the feeling of euphoria up or down, who decides what the level should be? The doctors or the person whose brain is on the line?
The authors were asking this question because of a patient who wanted to decide the matter for himself: a 33-year-old German man who had been suffering for many years from severe OCD and generalized anxiety syndrome. A few years earlier, his doctors had implanted electrodes in a central part of his brain’s reward system—namely, the nucleus accumbens. Electrically stimulating the patient’s brain had worked rather well on his symptoms, but now it was time to change the stimulator battery.