Disclaimer: FMA isn't mine.
They spent their whole lives running.
He ran away from things, she ran towards them.
After seeing the black lines that spread across her back, after watching her bury her father without spilling a tear, after watching her cry when she thought he wasn't looking, he left. And he hated himself for it. He'd left her alone in a rickety house, after robbing her of the one thing she had left - her dignity. He thought he had buried his past with his teacher.
She came after him. She had left behind the only home she had ever known, and joined the army. She'd come after him, running headlong into a war zone, her calloused fingers pulling the trigger because every second she endured brought him closer to him. Ishbal was no place for someone who knew little of the world, but she learned quickly. When her efforts had paid off and she'd finally caught up to him, he had realized that you couldn't outrun your past, no matter how hard you tried.
After watching his best friend's body lowered into the ground, after hearing his daughter cry for her father, after listening to the eulogist speak the final words for a man he knew nothing about, he left. He'd tried to forget that it was his fault his best friend was dead, six feet under. Drinking helped numb the pain, though relief remained just out of his grasp. He thought his dream had died along with the one man who knew him best.
She came after him. She had found him sitting in a bar that he and Hughes had often frequented, an empty glass in front of him. It was she who had driven him home, she who had fished his spare key out of the light by the front door to his apartment. It was she who had made him coffee and poured out the bottle of Vodka in the cupboard, despite his protests. When she had fallen asleep on the couch in his living room, he had realized that his dream wouldn't die, as long as there was someone left to fight for it.
After digging deep into a mesh of lies to avenge the death of his best friend, after discovering just how far the corruption in the government went, after finding that after all this searching, it was that miserable creature Envy who had killed Hughes, he left. He had ordered her to stay behind and run off into the labyrinth of tunnels that stretched under Central. Chasing a monster, while he was becoming one himself.
She came after him. She had ignored his orders and come after him anyway. She had been the one to catch up with Envy, had been the one to fight and be injured by that monster. And he had told her to stand down when she tried to save him from himself. And in the end, it was her that stopped him. Because if she died, because he was too stubborn for his own good, there would be no one left to fight for his dream. Because he knew she meant it. After all, if she had followed him this far, she'd follow him to her death as well.