Before the Ten Words were spoken, Moses did something remarkable — he spoke back to God. "The people cannot come up to Mount Sinai," he said, "because You Yourself instructed us, saying, Make limits to the mount, and sanctify it" (Exodus 19:23, Targum Pseudo-Jonathan).
The Targum Pseudo-Jonathan makes this reply almost legalistic. Moses is reminding God of God's own command. The boundary had already been drawn. A barrier of holiness encircled Sinai, and any Israelite who crossed it would die. So when the Lord tells Moses to descend and warn the people again, Moses pushes back: they already know. The fence is already in place. They will not climb.
The midrashic tradition sees in this exchange the paradox of Sinai. God says go down and warn them; Moses says they are already warned. And God insists anyway. Why? Because holiness is not a single warning. The fence must be repeated, remembered, renewed. A boundary that is only drawn once is a boundary that will be forgotten.
There is also something tender here. Moses, the shepherd, is defending his flock. They will not transgress, he tells God. He trusts his people even when God warns him not to.
The takeaway: the fences around holiness are not drawn because people are reckless — they are drawn because forgetting is easy.