When shop personel is not friendly, we tend to think that that person is not a friendly person, as a character trait. When I'm not friendly in the same situation, it's because I didn't get enough sleep, or because I had a fight with my girlfriend that morning. So in my case it's circumstantial, temporary, not my fault, in the other person's case it's the person's fault and permanent.

What you describe is our inner voice doing the same thing. (This is my personal explanation!)

Google for "inner voice doubt" and find out more!