Cashiers have a habit of telling me how shitty their day is going. I have no idea why. I usually ask how they're doing, just to be polite, and the usual response is, "I can't wait to go home."
If you don't want to know, don't ask! I can imagine that most cashiers will be having, at best, mundane days on the job and that their personal life is more interesting. So, you've got to expect the can't wait to get home bit!