...stealing a person's happy memories is *exactly* what a Dementor does; did Grindelwald find a /potions/ equivalent of their effect?! They're regarded as /the/ punishment for the worst crimes imaginable; it only hammers in that what Graves is going through is /excruciating/ to the extreme.

...and unlike Dementors, where simple distance and/or a Patronus is enough to reverse the effect, he won't ever get those memories back.