Why do people insist on being miserable?

Evening all! (Or should I say morning?)
I hope everybody is well and had a good weekend. Would love to hear stories of any crazy adventures!
Anyway, I'm back online this evening because, you guessed it, I have another little moan lined up for you.
Some of you may already know from my bio but for those of you who don't, I have a part time job as a waitress. I have worked as a waitress in a variety of places for just over 6 years now and have been at my current job for a month.
Different companies have different protocols and methods of doing things. But there is one thing guaranteed to stay the same wherever you work; the general public!
I am not one to look a gift horse in the mouth and I understand that without the public, I am without a job. However what I don't understand is why, when people come out for a meal, they insist on being miserable from the moment they walk through the door.
I am fairly sure it isn’t just the bringers of food who suffer; I assume it comes as part of any job when working with civilians.
But this is how I see it...
There are 3 kinds of moaners; complimentary moaners, bad-day moaners and miserable moaners.
The complimentary moaners are the people who hate to complain and apologise for flagging up their concern.
The bad day moaners are the ones who moan throughout and then apologise at the end of the meal and explain their situation. (Usually you can tell if someone is just having a bad day and that's fair enough, we all have them!)
But then we have the miserable moaners! These are the people who have it set in their mind that their meal will be terrible and the service non-existent before they even walk through the door. And the purpose of all their complaining? Frequently it’s because they expect a free meal!
If you come out to a restaurant with your family or friends or partner or whoever you go out to a restaurant with, go out and be happy!
Don't go out to rain gloom on other peoples' parade. You don't just ruin the evening of your waitress and your company, but of the tables surrounding you, too.
It creates a bad atmosphere and nobody enjoys eating and socialising when you can cut the tension with a pizza roller!
(Can you tell I had a rough evening at work?)

Thanks for reading guys :) all comments welcomed x