In John Donne's poem "Sweetest Love, I Do Not Go," the speaker explains to his beloved that he has to leave. However, he is not leaving his beloved forever—it is only temporary, which is why he does not want his lover to be too upset. The fourth stanza of the poem contains the passage in question:
When thou sigh'st, thou sigh'st not wind,
But sigh'st my soul away;
When thou weep'st, unkindly kind,
My life's blood doth decay.
Fundamentally, this passage is an expression of the intimacy between the speaker and his beloved. So close is their connection that when the beloved signs, she "sighs my soul away." This metaphor
indicates that the speaker is pained to see his beloved in a state of distress, because he experiences it deeply as a soul-level loss. Moreover, when his beloved cries, "My life's blood doth decay," a startling metaphor that equates the speaker's blood to the beloved's tears. This figure again suggests that the speaker feels a sense of deep loss and personal diminishment when the beloved is in a state of sorrow or anguish.
These metaphors also underscore the sense that the two lovers are connected even when apart, which is the speaker's central argument in the poem. In the speaker's imagination, the beloved'd expression of sorrow summon forth aspects of himself: his soul and his blood.
In the ensuing lines, the speaker makes the point that the beloved cannot truly love him as she says "If in thine my life thou waste." Thus, the speaker is asking that his beloved not sigh and weep. But the deeper suggestion of this stanza is that the lovers are deeply connected even when apart. In this sense, the lightly scolding stance—"unkindly kind"—belies an attitude of affection. The speaker develops this argument of connectedness further in the next and final stanza, when he asks that his beloved "But think that we / Are but turn'd aside to sleep."