Shakespeare's sonnets address the following subjects: friendship, love, jealousies, rejection, harmonious affection.

Major metaphors used: time devouring everything, seasons and weather mirroring human emotions, stars that stand for fate, free will, or providence, nature to show the passage of time and death.  So says Enotes:

Other important metaphorical patterns are linked to treasure or riches, corruption and disease, scarcity and abundance, and the effectiveness of procreation and poetry as means of immortalizing beauty and defying time.

In Sonnets 1-17, for example, the speaker uses the metaphors of conquer devouring time, the enemy of earthly beauty and love (Immortality through procreation).  The poet urges his friend to marry and eternize his beauty through engendering children.  Shakespeare's four favorite characters are: the poet-speaker – his friend – his mistress – a rival poet.  A major metaphor he uses is the love triangle between three of these four.