For this reason it is said to be a mystery

The Reading:

A monk asked Ikan, a student of Baso, "has the dog Buddha-nature or not?"  Ikan said, "Yes."  The monk asked, "Have you it or not?"  Ikan said, "I have not."  Monk: "All existent creatures have the Buddha-Natures; how is it that you have not?"  Ikan: "I don't belong to all existent creatures."  Monk: "You say you don't belong to all existent creatures.  This 'You', is it a Buddha or not?  Ikan: "It is not a Buddha."  Monk: "What sort of thing, in the last resort, is this 'You'?"  Ikan: "It is not a thing."  Monk: "Can it be perceived or thought of?"  Ikan: "Thought cannot attain to it; it can not be fathomed.  For this reason, it is said to be a mystery."

-Blyth's Mumonkan, Case 1

Discussion:

Ikan wasn't bothered by the question, "Who am I?"  Why is anybody bothered by it?