Some people in the wizarding world have soulmates. It doesn't mean the soulmates need to end up together or aren't able to resist the other, but it does mean that, regardless of anything or one, someone with a soulmate will always love the other in whatever capacity he/she can.

When Grindelwald kidnapped Graves, the two accidently found out that they were soulmates. Grindelwald's still an evil bastard and isn't above hurting Graves. However, the soulmate bond ensures that he feel some attraction towards Graves, and this comes out as possessiveness. Graves, for his part, hates how he feels and mentally rebels against the bond. He's well-aware of what Grindelwald is and doesn't plan on giving into him. But he still feels a need to be with him, which makes everytime Grindelwald hurts him worse.