Masochism
Ed started coming in battered and bruised a week after they started dating, but he didn't act ashamed or afraid; he almost flaunted his wounds. One morning Roy dragged him into his office and demanded to know what that bastard was doing, and Ed snarled back that he asked him to hurt him, he wanted this, and Roy asked if he was that broken, that the only way he could feel love was through pain.
And he saw, for a second, what might have been, when Ed whispered that if it'd been Roy, he wouldn't have needed pain to feel.