Economics. In the law of demand, "the size of marginal utility is negatively related to the quantity of consumption"? Why negative?

Economics. In the law of demand, "the size of marginal utility is negatively related to the quantity of consumption"? Why negative?

Let's take a look at the definition: marginal utility refers to the new efficiency increase brought by consumers adding a unit of goods or services in a certain period of time, that is, the increment of total utility
Then, suppose we eat steamed buns when we are very hungry
The first steamed stuffed bun is very satisfying. Of course, it has the greatest effect
The second steamed stuffed bun is also very comfortable, but it's not as good as the first one. The added effect begins to decrease
The third one is a little comfortable, but it's not as good as the second one. The added utility is further reduced
When you eat the tenth bun, you may be full. At this time, if you eat again, the increase effect may be negative
Negative correlation means that the more steamed buns you eat, the less effective it will be

The meaning of "law" in economic activities

In a narrow sense, it is the relevant law made by the legislature!
In a broad sense, it includes the relevant laws and authorizations made by the legislature, the laws and regulations made by the executive authorities and the rules and regulations made by specific departments!