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I love complainers because they care enough to let you know that they are unhappy.
They are presenting you with an opportunity to make it right or do you best to do so. Most times people want their concerns validated, their worth as customers and as people reinforced, and that they are taken seriously.
I get a lot of my new business from my competitors screwing up these opportunities. Even if I can't help them now(outside of my expertise or my schedule), they come back because I showed them that I do care about them and their business.