I'm not a big proponent of "the customer is always right", however, there is certain truth that business owners need to face up to. Let's face it, nobody enjoys the relaity check when a customer complains. It may be true. It may even be unreasonable. It might just be a bad day for that customer and you are right there in front of them, just at the wrong time, so, you'll have it. Nevertheless, the most important thing about your business is to work on minimising all the things that may open you up to that possibility. Of course you won't reach perfection, but, you must aim towards perfection. Is your staff up beat? Are you holding enough of the most important stock? Are you paying people on time? Are your premises appropriate for your level of target market? Simple things, like is there soap or toilet paper all the time? Do you do things on time? Are you approachable?
Most importantly, are you prepared to change any of the above if the answer is "not always"? Addressing all of the things that your customers give you in the form of complaints or concerns is a gift. They are actually giving you ideas on how to get better. Thank them and get right to business in fine tuning your business to work towards the never achievable perfection.