I've encountered one too many evil customers.
  1. Because they're a person. With feelings.
  2. Because the customer is pretty much always wrong.
  3. Because not everything is as big of a deal as you may think it is.
  4. Because they don't want to be at work most of the time, so treat them with respect.
  5. Because if you're nice to them, they will be nice to you.
  6. Because causing a scene embarrasses you, not the employee.
  7. Because everyone makes mistakes from time to time, even you.
  8. Because they're a person with feelings. Doing their job.
    Can't stress this more! Be reasonable people.
    Suggested by @jessicaz