Lots of people have fallen in and out of love with Edward Elric. And he knew that, he always had.
First there was Winry, and maybe that one lasted the longest. He loved Winry, too. She was his first and only friend. She stuck by him through all his mistakes, triumphs, and tragedies. She was beautiful and kind and smart and funny. But he wasn't in love with her. And when she asked him, after he'd returned from across the gate if "ever in his entire life he held a shred of passion for her?" he'd answered almost detachedly and quite bluntly, "no."
Then there was Mustang. They were alone in his office, and they were fighting. Flinging words back and forth like tennis players whipping rubber balls around in their final set. Edward didn't realize they were getting closer and closer until their feet were almost touching. Mustang paused mid-rant and just looked down at him for a moment. Then he was moving and suddenly his lips were on Ed's coaxing his mouth open. Edward didn't move away, but he didn't exactly contribute either. Mustang pulled away and just stared at him before whispering, "I love you, Ed." When Edward didn't respond, he sighed and went back to sit behind his desk while Edward fled the room.
There were others, too. There was Rose, Russell, Noah, and Alphons. He loved all of them, really, he did. But he didn't fall in love. He couldn't, because saw what love could do. He saw what love did to people. He saw his mother wait, wait years, and then die. She died because she was in love. So Edward couldn't fall in love. He wouldn't let himself become so vulnerable, to step off the last step and reach that unidenifiable, unsuspected, and completely impossible to predict feeling. He didn't need it or them. He didn't need to fall in love. And even if he did, he couldn't. So he'd just have to thrive without it, he always had, and he always would. Afterall, he had more important things to do. Love could wait.