“Sonnet 129” features three main comparisons. The first one is a metaphor in the opening quatrain: “Th’ expense of spirit in a waste of shame / Is lust in action” (Lines 1-2). The opening metaphor introduces the poem’s main thematic concern, The Dangerous Nature of Lust, while connecting lust to the moral concern of shame that the poem focuses on as it progresses.

The second comparison is the simile between the lustful lover and a baited fish or animal. This comparison takes place midway through the second quatrain and continues into the third quatrain. Here, the speaker demonstrates the animalistic nature of lust by comparing the lustful lover to an animal lured into a trap. It also demonstrates the way lust overpowers logical thinking, reducing those who fall into lust to the basest emotions humans can experience like desire, greed, instinct, and possessiveness.

The third comparison comes in the final couplet, when the speaker compares the consummation of lust and its aftermath to a moment of heaven and an eternity of hell. In using this spiritual imagery for the sonnet’s final contrast, the speaker reinforces his harsh conception of lust as something that leads only to pain, disgust, and disillusionment.