How Hard is it to Just be Nice??

I'm at home today (yeah, for a three day weekend) and noticed something. I went to Subway to pick up a sandwich for lunch, and as I'm ordering, a man came in. He waits his turn to order but as soon as the clerk asked for his order he turned into a real jerk. Who goes to a sandwich shop and doesn't expect to be asked "which bread"? If someone answers "just regular bread" would you not ask for clarification? Then when he's asked about what kind of cheese, he gets even more agitated, grumbles something under his breath, and has to go back outside and ask whoever is with him. It all just made me very uncomfortable. All I could do was tell the clerks that I hoped they had a good day as I was paying. Maybe it's my upbringing as a child of restaurant owners that makes me sympathetic to all service workers. Whether it's at a restaurant, a grocery store, Wal-Mart, etc..., I always try and "think" before I speak. I never want to be "that" person that is talked (or complained) about after I leave that establishment. I can't help but think "if you're in a bad mood" or if your day "isn't going that great"---it's NOT that person behind the counter or cash register's fault!!!