Why do some People Complain about everything

I can list a number of situations and scenarios in which people tend to complain about everything.
Why do people complain about everything?
There is not one definite answer for that question. Instead, there are a whole bunch of answers. I have been in situations in which people have complained and put up a temper tantrum. However, those situations are not the same. Hopefully, answers can be given by explaining the situations that I have to deal with.

SITUATION ONE:

There are people that are virtually impossible to satisfy. My grandmother happens to be one of those people. She tends to get on my mother’s nerves. When my grandmother is not around, my mother often complaints how nothing is ever good enough for her. My grandmother is stubborn and does whatever she pleases for the most part.
It makes things difficult for me and my mother for the most part. However, my mom just lets it roll on her back.

My mom told me that I have to understand that my grandmother is very old. In some cases, age could be a factor on why some people complain about everything. It is an interesting thing. When we are young, we are considered to be liberal. However, things change over time. When we become older, we get a bit more conservative. With all the changes in the world, it could be one reason that some people complain about everything.

Perhaps it is my grandmother’s old age as a reason she tends to complain about everything. She is over eighty years old right now.
However, she does seem to be pretty healthy.

SITUATION TWO:

My aunt has a tendency to complain about everything. This is for two reasons. The first reason is that she is constantly concerned on how other people think about her.
If they feel ill-will towards her, my aunt will go nuts. She can be very self-conscious for the least. Because of this, my aunt tends to complain about everything.

The second reason is that my aunt is very demanding. Her demands can be very unreasonable. She always has to have it her way. If things are not her way, my aunt will complain. The people that insist on having it their way all the time tend to complain all the time.

SITUATION THREE:

There are manipulative people that complain all the time about everything. There are two situations in which I had to deal with such people.

The first person would be my eldest cousin. He is an extremely manipulative person. My cousin managed to get just about everything he ever wanted. As a result, he never really understands the value of things he is given. He expects everything to be given and handed to him on a silver platter. However, it does not happen anymore. As a result, he complains about everything and puts up a major temper tantrum.

The second person would be a problematic member I had to deal with on a role-playing forum I used to have. His case is similar to that of my cousin. However, the person is incredibly predatory. The reason he complains about everything is that he wants everything his way.
He tries to force his views and ignorance upon other people. When people do not buy his views, he complains. In this case, he lacks the maturity to structure complaints properly.
Also, he constantly complains to constantly make trouble for me and my other staff members. In this respect, he is very vindictive and contemptuous.

These are some reasons that some people complain about everything. For each person that complains about everything, there is a different reason. In the case of my grandmother, it was a combination of old age and things not being good enough for her. In the case of my aunt, she was a self-conscious control freak. For the other two, they are manipulative and want everything given to them.