A key theme in Shakespeare's sonnets is the passing of time and its inevitable impact on the beauty of the person addressed in these poems. The speaker seems concerned and deeply saddened by the many different reminders of the inexorable nature of the passing of time, which leads him to consider how the beauty of his beloved will likewise fade and diminish, just as in this poem he beholds "the violet past prime" and the "sable curls all silver'd o'er with white." The changing of the seasons as we move from summer into fall indicates the symbolic "death" of the beauty of nature. You might want to consider the images of death that we are presented with: the trees that are "barren of leaves" and the "brave day sunk in hideous night." References to time emphasise the way that all things die and pass into blackness, which of course makes the speaker realise that his beloved to "among the wastes of time must go." The only defence mankind has in the face of death is to procreate, and therefore create images of ourselves that can stand against time a little longer:
And nothing 'gainst Time's scythe can make defence
Save breed, to brave him when he takes thee hence.