Working in retail, I still remember one of my worst customers. He handed me a quarter and what looked like a single one dollar bill. I said, “Sorry, the total is two twenty-five.” He pulled apart two crisp bills, which I didn’t notice were stuck together, and slowly counted, “One…two. Do you speak English? Do you know math?” I was fuming, but I said nothing. I was, however, short with everyone else that day, until a friend asked, “what’s your problem?” The problem was: I let that jerk turn me into an jerk, too.

Source: How to Avoid Turning Into a Jerk When You’re Surrounded by Jerks