"You're losing weight, and you don't need to. Have you been eating enough? "

Graves would find recovering from his capture much easier if his job didn't involve interacting with Grindelwald now and then, and if the man didn't act like he cared.

Note: Lima syndrome is when the captor bonds with the captive. That being said, just how much Grindelwald actually cares about Graves and how much of it is manipulation is up to filler.