Why do we all say your last name now, not yours?

Because the word "your surname" has been widely used in ancient times, it has been changed since it spread to modern times. If you forcibly change it to say that your surname is not good, it will be a bit evasive. Moreover, feudal superstition has existed in China since ancient times, including now. Building a house and getting married depends on the date and the horoscope, not to mention the fact that it is related to one's own name, because some places of surnames can be said to be dead and unlucky.

China's culture is profound, and there are many uses and meanings between words. For example, in ancient times, surnames were separated, in addition to names, characters and numbers. But we don't think modern people are that troublesome. Generally, they have two or three or four names, and most of the time they have a nickname at most. Last name+first name make up a person's name. This number is a nickname at best, not even a nickname.

By the Qin and Han dynasties, the word surname had been combined into one, and the meaning of surname was what your surname was. Later, the names were merged, and it was not until the modern number began to be used. People are calling each other more and more succinctly.

Moreover, the homonym of "teacher" is death, and its meaning is not very good. Let's imagine that two people we don't know have just met, and one of them says that you are your last name, which sounds strange. Everyone just uses different words on different occasions. For example, some villages or towns are generally people with the same surname and have genealogy. Generally speaking, they say X's genealogy, but they don't say X's genealogy. It seems that the ancients also understood that words were not fluent.