Figurative language --- any "figure of speech" that is not literal in meaning -- is enriching to literature, nowhere more so than in love poems. Figurative usages are like the spices you toss into a spaghetti sauce to make it special; you could do without, but the end result is the poorer for it. In fact, the simile "usages are like spices" is figurative language. Love poetry uses similes, metaphors, apostrophe and personification, among other literary devices, to create imagery that clarifies and enhances the poem's emotional effect.
To write good figurative love poems, study the masters. Shakespeare's "Sonnet 18" compares his lover to a summer's day, with "thou art more lovely and more temperate." This is a metaphor, with the woman as the day. He promises her "thy eternal summer shall not fade," which is both hyperbole and apostrophe, as he exaggerates to one who is not present. Shakespeare ends with "so long lives this and this gives life to thee," a personification of the poem as an eternal life giver. Each usage intensifies the emotional truth of the poet's commitment to his beloved.
Emily Dickinson, in "Poem 1260," uses figurative language for a darker purpose, sorrow over a loved one's death. Her emotions are overwrought, so her hyperbole is grand: The lovers found each other, a "discovery not God himself could now annihilate." The poet's pain is so intense, she uses personification to reprimand the departed and introduce a paradox: "You, who were Existence, yourself forgot to live." Figurative language is sparing in this poem, making its occasional use all the more intense, as the truth of the poet's absolute loss expresses itself in absolutes.
Figurative Language for Truth
The best rule in using figurative language in love poetry, according to Poetry Foundation's forum "How to Write Love Poems," is to work toward "not the poetry but the heart," to use figurative language not because you think it's required but because your emotion is so strong that literal words are not enough to express it. Your beloved is "a glowing ember," kissing her is like "melting into chocolate," her scent is "a gentle stroke on the cheek." You use metaphor, simile or personification not just for effect, but to intensify your emotional truth.
Metaphors Are a Trap
The Poetry Foundation forum warns that the use of tired similes and metaphors is the worst trap in writing love poetry. Stephen King in "On Writing" reiterates this, begging young authors to avoid trite figurative clichés such as "my love's like a red rose." Both King and the Poetry Foundation advise writers to read and write a lot in their preferred genre, thus rediscovering and reinventing similes, metaphors and other usages that express love figuratively, intensely and truthfully.