Sonnet 106 is a “fair youth” poem, in which Shakespeare compares the beauty of a fair youth to other kinds of beauty and finds the former to be superior.
Here as elsewhere, the “I” of the poem is the speaker, who is not necessarily Shakespeare himself though can certainly be envisaged as such. The “you” is the object of the speaker's affections, a fair youth whose remarkable beauty is to be celebrated and lauded to the skies.
In this particular sonnet, Shakespeare dismisses the efforts of previous poets and artists in rendering beauty. The praises they lavished upon “ladies dead and lovely knights” merely foreshadowed the extraordinary beauty of the youth to whom the sonnet is addressed.
Yet Shakespeare, despite this criticism of his poetic forebears, does at least have the honesty and the humility to recognize that he, too, cannot adequately encapsulate the fair youth's beauty. He may have “eyes to wonder,” but he, like everyone else, cannot put the youth's beauty into words.
Poets of the past would not have been able to sing the youth's praises, and neither can the speaker himself. What's more, he lacks the requisite skill to offer him the requisite praise that his extraordinary beauty demands.