When dealing with customers, there is one thing that is so important to remember, yet so easy to forget. Customers typically do not act or react based on you as a person, rather the reaction is based on you as the solution to their issues. When a customer is not satisfied, or has a complaint, the customer is not necessarily speaking to you as a person, but speaking purely based on whether or not you can resolve the issue they have.
When a customer is frustrated, they are looking at you as the answer to the problem, and that is the root cause of their frustration.  If you are not able to resolve the issue, or you do so in a not so timely manner, the reaction from the customer more than likely will not be a positive one, but you must remain positive, and never take it personal. If you take it personal, it will negatively effect your overall outlook, and ability to resolve the issue. If you choose to stay positive, and not let the negativity of the customer deter you, the positive outlook will give you a better chance of not only resolving the issue, but changing the mood of the customer as well. If you take it personal, and react to negativity with negativity, there is very little chance of bridging the gap between where you are currently at with the customer, as well as the issue, and where you need to be.

Regardless of the professional relationship you have with a customer, when it comes to issues, the customer bases their actions or reactions strictly on a business level, not a personal level. If you keep this in mind when dealing with clients, it will help keep you from falling into the trap of taking it personal. The end result will keep you positive throughout, and in turn displaying to the customer that you in fact are the solution to their issue.