Roy Mustang pushed people away.
He couldn't help it; it was just the way he was. He would allow people to get closer until they crossed the line and were too close, then he would push them away. Friends, family, partners, some how he had never learned to stop people getting too close.
With Edward, it was different. With Edward, the 'too close' line was way past the 'too important to lose line'
But he found out too late, by the time Roy realised Edward had crossed that line, he'd already pushed him away.