Why do northerners like to call themselves Laozi when they talk?

This is where you are wrong. I'm from the north, too. I admit that people here are rude, but as you said, this situation is rare, even a few years ago. It's rare for us to call ourselves Lao Tzu now. Because everyone knows that it is a lack of quality. Even if there is, it should be entertainment or joking. It's rude for northerners to joke. If you don't know, you will really think it is swearing or fighting, but don't get me wrong. Not all northerners are as bad as you think. There are also stupid and civilized people!