Some people get scared of complaint or become defensive. I like to embrace them. The main reason I value a complaint is because they give me a course of action.
It may be something you may have not been aware of that needs your direct attention. The person giving you the complaint is really offering you a gift. The gift to make things right for everyone else who comes through your doors.
See to resolve what, who, and/or why is cause for the complaint. Be grateful to the person letting you know because they trust you enough to care by saying something when most walk away annoyed. Most will gripe and accept it as it is. They are holding you to a greater standard and keeping you accountable.
Remember it’s better to have a complaint and embrace it. Then have quiet mumblings and dissatisfaction. Most important say, “Thank you,” to whomever is bringing you the complaint.