Working with the public for most of my adult life has ruined me. I want to believe that there is good in all people but after my experiences, it is hard to feel that way. The public can be cruel. People seem to nit-pick about even the smallest of details. I have found that some folks will complain about anything just because they can.
Negative people feed off of negative energy. They think that if they are going to be unhappy, everyone around them should also. These people will go to any lengths to drag down everyone around them. They have no respect for others. They don't care if it is your fault or not, they will still try to make you feel like it is your fault.
Many people who complain do it to get a rise out of others. They like to see what buttons they can push and how quick they can push them. They may also like getting free sympathy and pity. They want to play the victim. They like the idea of others feeling sorry for them. They feel like the whole world owes them something.
I have seen some foul actions from people such as these. There are people who will complain just to get a free product or service. They don't care who has to pay for what they get; they just want what they want when they want it. I have seen people complain over watered down gas, hair in prepackaged food (such as crackers), and losing lottery tickets. It is as though they just want a reason to fuss and fight.
I hate to say that there is no hope for these people but I know that it doesn't help to reason with them. If someone has their mind made up then there is nothing you can do to sway them. Nothing makes someone who is dead-set on being a pessimist happy. For many people complaining is the only way they know how to communicate. They are so used to bullying others with words that they may not truly understand how to get their point across in a positive manner.
Although, the customer (or the one that is complaining) isn't always right, the only way to deal with this type of person is with a grain of salt. It is best not to let their complaints get to you. You can try to come to an agreement that will make all parties happy. If this doesn't work, ignore them. Regardless, do not give them the upper hand. Never let them have the satisfaction of bringing you down too. Complaints are a pain in the butt yet, they make the world go around. If everyone were always happy, the world would be an uninteresting place.