Most things are relative.

Probably an internal, therefore subjective, dissatisfaction with their life. To wheel out the old cliche, if your happy in yourself you don’t need anything else! External projections such as belief or acquisition will always lead to unsatisfactory situations.

Go tell it to the homeless

What I call "keeping the drama going" sad in a very negative way

Some people have a drive to find try to improve things and find flaws, or things that could be better. Some people are just happy with whatever they have and don't question things much.

Everything is relative to our current point of view.

Some people just aren't happy unless they have something to be unhappy about.

Is the level of complaining improving?
If it is, then there is no need to complain about complaining. If it isn't, then that's one thing that isn't improving, thereby providing something to legitimately complain about...

My ex was like this, always complaining about something.. hence why she is an ex. For her, and I suspect many others, it has a lot to do with how a person is raised and their experiences in life. Her mother is a complainer and from what I heard, her mother's mother was one too. My ex has a past that I am pretty sure created a narcissistic/borderline personality disorder in her as well, which gives rise to a lot of negativity. When you're so used to being told negative things while having negative things happen to you, it'll rub off on you.

I also believe that when a person experiences traumatic event, they either disconnect from their ego to avoid the emotional toll or connect more with their ego to over compensate the emotional damage. For some, this is a permanent change and for others it's temporary. Those with the bigger connection to their ego will complain more as there is less to appease their ego.

So I'd say big egos and a person's nature/nurture play a huge factor in the amount of complaining they do.

People many times complain with a belly full. It's also part of human nature. Silly but true.

You're right. Women and people of color sometimes have the right to vote; what more could they want? Beyond that, I'm not really sure what has improved?

I think H.sapiens evolved to exist in an enemy-rich environment, so if we don’t have animal predators or tribal competitors or food scarcity to deal with, we fill the void with challenging beliefs. It’s our nature to be fighting something.

There are always those that are in the "don't worry, be happy" camp. Unfortunately, things are not getting better for very many and that number is going up. Sure, if one considers material comforts things have advanced beyond our wildest dreams. But the really big picture item like the crash of the environment and our political system things are going downhill very fast. No matter how 'wonderful' things are right now (for some) many of us look to the future and see where we are headed. It's called reality not denial.

Not all the people, just the losers. Lazy, unemployed, nagging complaining people.

I'll give a go at this and say a lot of it comes from the ideas of a heaven. It's something that's better than we now experience so no matter how good we have it, there is always something better waiting for us. If we were taught by parents and teachers really how good we have it then possibly people could change and enjoy what they have.

I think a friend of mine said it well. He said that as people's situations improve, so (usually) do their expectations and standards for what they feel satisfied with as far as the conditions of their life. So their level of satisfaction remains about the same.

There must be some deep psychological reasons that people judge the world to be ugly, unjust, threatening, hateful, etc. I think that to hold grievances and be angry provides a certain kind of ego gratification— provides a cause, a reason for existence. And by designating certain groups as idiotic, ignorant, selfish, self-righteous, etc. one creates an enemy to fight against, thereby casting oneself as a great enlightened savior, intellectually and morally superior.

So yes, some people will always find something wrong with the world. IMO the true and correct perception of reality is one of great beauty, love, awe, reverence and respect for all people and for the overwhelming mystery of existence.

I think because we evolved to deal with scarcity as a default state that we don't know how to handle abundance. There is something about human nature that just assumes that it is a lie, and actively rejects it. Never before in human history was the possibility of the virtual elimination of all human suffering ever as close as it is today, but people are just as sad and angry about it, maybe even more so if its so little that people are not desensitized to it the way they used to be.

Could it be that we live in a world of artificial scarcity, and that this makes many people feel compelled to gripe?

Complaining seems to be the natural, default state for whatever reason.