Why does the poet call black night death's second self?

Just to make it clear, this line actually comes from "Sonnet 73." This particular poem is a rumination on old age, and largely consists of a collection of appropriate metaphors. So for instance, the speaker refers to "twilight" and the sunset that "fadeth in the West." The speaker is preparing for what he perceives to be his imminent demise, and wants the young man whom he addresses not to be sad, but actually to take heart from this. The hope is that the young man's love will gain strength from a greater understanding of death, fortifying him against the inevitable sorrow that will be occasioned by the speaker's passing:

This thou perceivest, which makes thy love more strong, To love that well which thou must leave ere long.

The speaker describes the dark of night as "death's second self." What he means by this is that night is just like death in that it is a time of darkness and rest. The difference, however, is that the darkness and rest of death are permanent. Death also takes away the light of life just as night takes away the light of the day. Given the close imaginative connection between the two, we can see why the speaker invokes the black of night as a metaphor for death.
https://www.poetryfoundation.org/poems/45099/sonnet-73-that-time-of-year-thou-mayst-in-me-behold