The tablets were G-d's handiwork and the script was G-d's script, engraved by G-d Himself on the tablets.
The Written Torah is written on parchment, with ink. It thus comprises two separate components: the message (the words written in ink) and the medium (the parchment upon which it is written). In the case of the Oral Torah, too, there are two separate components: the words (the message) and the person who is studying them (the medium for their articulation). Although we may become emotionally and intellectually involved in our studies, they nevertheless remain a peripheral addition to our essence.
The Ten Commandments, however, were engraved on the tablets themselves. Rather than two separate entities, there was just the stone itself, and the commandments were engraved onto it.
When something is written, it can be erased or scraped off; when it is studied orally, it can be forgotten. In the case of engraving, however, there is no way of separating the writing from the stone. It can be covered up, filled in, or additional parts of the stone can be chipped away so that the writing becomes illegible, but it cannot be erased or removed. The medium has become one with the message.
This is how we must approach the Torah. When we study the Torah, we should be so lost in it that all that exists for us is the Torah itself; the medium, the message, and the recipient of the message all merge to become one.
With this approach to the Torah, we can never completely lose touch with it. The sands of time may cover the engraving, or the temptations of the world may make it hard to decipher; yet, throughout it all, that connection is there, and cannot possibly be revoked. This is the message that G-d imparted by carving the commandments in stone: "The Torah, you, and I are all one, and this unity can never be severed."
—From the Kehot Chumash