Why does my mom nag so much?

She is constantly nagging to me about the stupidest things. We just drove across Houston and it took 2 hours with traffic, and she complained the whole way about i am not nice to the kids in my class, why should i be nice to them if they are not nice to me? The other day there was a bad storm coming trough and our back door blew open and she blamed me for not shutting the door when she was the last one to go through the door. This mourning we where on the beach and she started yelling at me because it wasn't setting the canopy up right and how i had gone into the ocean without sunscreen. And like 200 people started looking at us. I told her that she was nagging, and she nagged about that to! As i'm typing this, she's nagging. She is so busy complaining that she does not notice that I am on the computer typing.