A Friendly Reminder

How come whenever someone sends you a friendly reminder, the tone of the reminder is never friendly?

I had another scary run-in with a librarian yesterday.
I went to the library to use the computer to pull some sources for my research paper. I gave the gal at the desk my card and she assigned me to a computer. No big deal, right? Wrong. I was assigned to #27, which one would think would be located next to 25 and 26, right? Wrong again. So, I went back to the desk to look at the little map of the computer lab. I was looking at the map and located my computer, when all of a sudden, a different librarian pops up out of nowhere from behind the desk. She said in a tone that sounded like she thought I was doing something illegal, "Can I help you with something?" Well, she scared me, and I could hardly talk, but I was able to get out in a wispy-sounding voice, "No, thank you. I found it." Then, I almost backed into a bookcase, before running into the room where #27 was located.
CREEPY!