Premise drawn from the disturbed maze that is my dreamworld: A brain enhancement that leads you into a state of unending "bored euphoria". This is a rough translation, shapes, concepts, and words are all deeply slurred when asleep, there's things I still almost remember but can't actually explain. The cause doesn't matter (and indeed borders on unexplained), it can be surgical, cybernetic, magical or even memetic, and has to be a true, objective improvement over their prior mental capacity. No empty hedonist garbage or chemical flooding. We should also assume that this person is of sound morality and has a reasonably high mental if not emotional health.
What I've figured out, arranged in order of most to least plausible:
Possibility 1: The upgrade simply enables one to ignore their own suffering as much as they want without erasing it through willpower.
Possibility 2: The individual can't not pay attention towards an internalized Truth, while somehow still able to live "normally". Quotation marks as misery is thoroughly normalized inside modern society.
Possibility 3: They have achieved complete awareness of all the misery and suffering that they can and will face thus being able to completely transcend the need for emotional worry.
Possibility 4: This person has figured something about the Truth of the universe so total that simply knowing it to be the Truth disconnects them from any finite misery. I don't think that this is remotely possible.