There, in the side of the Mirror, near the lower right end, is a little, precious, hairline crack.
The Mirror, once told about Achilles (had It been able to truly refer to Itself as sentient, and understand irony beyond the fact that It thinks without a brain and lives without breathing), referred to it as Its heel. It's little imperfection. Its maker's mistake.
The Mirror's glass is perfect. The glass is the important part. While a great amount of care and arithmancy went into crafting Its frame, the glass was the artistry. Was the crowning jewel of the piece. It glimmered and glistened and wreaked havoc on the lives of men for centuries. It was Power.
Glass must be held, however. A Mirror is hardly any use to the sorcerer who doesn't stand to look through It. And bending down, for the likes of Merlin and his apprentices, was too much of a surrender. (And Merlin surrendered to no one but his Lady.)
And so, in transit between existing and living (without breath or heat or blood), the Mirror was damaged.
This, It thinks, is the reason It is as It is. An object that knows without knowing what it knows, and object that wants without having the capacity to want.
At least, It smirks (without lips or a mouth), by the standards of men.