Habit
Fighting was their way of communication. It had started as a habit, Roy liked to tease Ed and Ed liked to annoy Roy and so it had continued until eventually it became more than a habit. It became a way to test each others limits. You needed to test the waters so to speak before you went any further, didn't you?
With two people who both have serious issues with trusting anyone else but themselves, it was unavoidable that they both needed to know where they stood. Whether the other would leave if they did or said something wrong. Whether the other would get tired after being exposed to the worst traits of their personalities or the darkest secrets. They both had many. But Ed thought that if Roy stayed after all the fighting, shouting, kicking and screaming, he might just stay for good. And Roy thought that if Ed didn t run away after finding out what was hiding in the darkest corners of his mind and dealing with his moments of self loath, he must be serious.
When Ed was sure about Roy and Roy trusted Ed, the fighting became a habit again. It kept their relationship alive and neither of them could have imagined a day without arguments or power struggles about the most insignificant things. It made their relationship intense, passionate, equal and perhaps a little bit twisted. But Ed had learned to walk out in the heat of a fight; he walked around the block, came back and was able to continue the fight without saying the most hurtful things. After all, fighting was a habit they both liked, it was not meant to hurt, not anymore at least. And Roy had learned to talk to Ed instead of shutting down and causing more arguments, and found other things to argue about - Ed s bad habit to write all of his reports late for instance.