Disclaimer: FMA isn't mine. :(
Sometimes he did the things he did, just to see if he could get some sort of a reaction out of her. Sometimes he was a bit more reckless or rude, angry or harsh, just to see if he could make her display some emotion other than indifference. He knew just what would irritate her the most, just how to appeal to her softer side, after all those years spent together in her fathers' house.
Rarely did it work.
When it did, it was an accident, on both of their parts. She had slipped for a moment, and he had overestimated himself. Or the other way around. It was the times that he didn't try that gave him what he was after. And even though the reaction was always partially the one he was looking for, he always wished it hadn't happened in the first place.
There was the battle against that homunculus, Lust. He'd almost died. He should have died, by all natural laws. And her mask had slipped, shattering on the cement floor. Even though he would have given almost anything to know that she actually cared whether or not he lived or died, he hadn't been prepared for exactly how much she had cared. He hadn't realized how much it would hurt.
He would have given anything to turn back the clock, to make is so that it hadn't happened. So that she wouldn't have had to realize either.