Apparently, Ahaz is not a believer. The admonition from Isaiah is not sufficient. In this particular case, the Lord volunteers a sign so that Ahaz can believe. Ahaz is not asking for the sign, but rather the sign is being offered to him. Isaiah tells Ahaz that, in essence, he may choose his sign, for it could be anything "either in the depth or in the height above - references meant to be inclusive of all possibilities.