If we think about how he taught her how to use her new body (which was very strict), I think there was a lot of yelling involved. He has scenes where he yells at her for not shutting up and following his instructions immediately during surgery. He takes his work very seriously and I assume that he'd think if he was gentle with her or forgive her mistakes even slightly, she'd make a fatal mistake that'd cause problems. Unfortunately, he's not very patient with her.