In a bussiness tragedy,why they say (Treat your customers like a king but don't let them be king on you?)

business can be small or big, as everybody says that customer will be the king, but if your confident about your product or services you are selling then it does not matter that on those saying.  If your customer feel that you are giving more friendly response to customer then you will certainly see that your profit is declining.  I mean to say that don't let the customer know that you are giving them too much response.  Be in the business with full confident on what you sell or service.  And of-course some or more lie will be there in any business.