These two sentences skillfully use the rhetorical device of pun. The word "silk" in "Spring silkworms must weave until they die" is homophonic with "thinking". You miss each other like silkworms until you die.
2. Metaphor
The image of "Spring Silkworm" has many metaphorical meanings. First, it is the association between the lingering attachment of human beings and the endless spinning of silkworms, from spinning to "death" to the eternal love of human beings. "And the candle will cry the wick every night" is a metaphor for the endless pain of not being together, as if the candle burned to ashes and the wax tears ran out. Taking candles as a metaphor, we don't simply compare pain with wax tears, but further use "turning to dust" to reflect the painful feeling of life. Want to stop lovesickness, want to stop sadness, unless you die. For this love, the poet would rather entrust his life to endless thoughts and sorrows. What persistent love!
The two poems take spring silkworms and candles as metaphors, showing the poet's deep affection and endless yearning for his loved ones until death, implying symbolic significance in moving images, making them surpass love and having eternal significance in praising persistent life. Because of this, this couplet has almost become a household name in Li Shangyin's poems. People often use these two poems to express their loyalty to love. At the same time, the persistence of "Silkworm to Death" and "Candle to Death" has aroused people's multi-angle association with these two poems, and now they are often used to describe the dedication to the cause.