"How other people react is their karma, how you react is your's," my yoga teacher said a while ago. When the supermarket cashier is grumpy or the boutique salesperson is curt I find it unpleasant and it makes me uncomfortable. When someone is angry I tend to take it personally and think the anger is directed at me. Most of us react that way. But it helps to put emotional reactions into perspective.
Imagine your boss just reprimanded you for submitting your report late. When you step out of that office you feel pretty lousy and might snap at the first person that comes along. Remember, though, the one who feels lousy is you, not your coworker who happens to walk down the hallway. If you snap at your coworker she probably thinks that she did something wrong, when instead something happened to you. See how intertwined we are?
It helps so much to be aware of our emotional reactions in order to diffuse them before they cause damage. How about taking a few deep breaths, going to the bathroom or the coffee station for a brief break, or being honest with your coworker and saying "My boss just chewed me out and I kind of feel lousy right now, do you want to have a cup of coffee with me?"
So back to the top. You can quickly and easily do a whole lot of damage with a crappy reaction, or you can choose to prevent a whole lot of damage by diffusing negative emotions.