It frustrates me when people conflate business and personal matters. Myself, I tend to view almost everything as business and rarely take anything personally. One of us didn't get what we wanted this time -- maybe it was me, maybe it was you -- but that's ok. It's very likely that we'll do business together again in the future, so let's not make a big deal out of this interaction. I tend to find it unproductive to get angry about many things. I try to make it worthwhile to interact with me, but if people don't want to then that's fine, someone else will. Likewise, if I don't want to interact with someone in a given circumstance, no hard feelings, maybe next time. Maybe I take this too far sometimes by treating what should be personal matters with a "just business" attitude.
The flip side is when people treat business situations as if they're personal, and it's why there are all sorts of maxims advising against doing business with your friends and family. "Don't mix business with pleasure." "Don't sh** where you eat." Many people take business far too personally, and if an arrangement doesn't work out for business reasons they take it as a personal insult. You don't want me, your own cousin, to paint your house?! But I'm a painter! You must hate me!
Some people have claimed that everything is business, and others have claimed that everything is personal. That leads me to think that everything is both, mixing in an amorphous manner that's impossible to quantify. Since I don't like unquantifiable messes, I like to treat everything like business, which is probably why I come across as insensitive. Maybe there's some middle ground.