Sometimes their conversational style is so different that it causes unnecessary stress.
He might say "Get me these reports by 5pm". Direct and to the point.
She hears "Finish these damn reports by 5pm or there'll be hell to pay". Because his tone and pitch and possibly even his speaking speed might be fast, she hears anger and impatience.
He thinks she talks slow and asks confirmation often, so he gets irritated and want to double-check her work , which undermines her confidence and trust and leads her to her conclusion that he's mean and angry.
And on and so forth.
If neither party is willing to be open and talk it out, someone's better off moving.
I read this article about having happy employees:
http://boss.blogs.nytimes.com/2010/03/09/the-secret-to-having-happy-employees/ is to fire the unhappy ones.
For whatever reason, they don't fit in. It's USA, you can fire them. Because you cannot make EVERYONE happy.