No one wants to feel as though they are being shoved off, that their concerns are unwarranted, immaterial or simply stupid, so a diplomatic response is sometimes what you may have to give them. Possibly, the best way to understand the customer is to put yourself in their shoes. They do not know what you have had to go through in that day or week which has led you to react the way you have. Their concerns are only with themselves. It may sound impolite, but when anyone goes to a business to purchase something then they expect a relationship which will eventually get them what they wanted.
There are also things not to say to a customer. An article from Inc.com is about the top 5 things not to say to a customer.
You can read the article here.