A man sighs and weeps, and the woman he loves laughs at him. That is the whole situation, and the poem makes it comic and tender at once. The speaker is a poet whose sorrows amuse his beloved, yet he admits that he "could not live if she should die." She, for her part, "says she loves me till she dies." The central idea is that love is rarely an even exchange of moods. One person can be solemn and the other amused, and the mismatch does not weaken the bond. It may even be part of it.
The first line already shows a gentle self-mockery. "At poet's tears" has no article, so the speaker becomes a type, a stock figure of the weeping versifier, seen as she must see him. Then comes a small paradox: these tears are "sweeter than any smiles but hers." Even while complaining about being laughed at, he pays her a compliment, because her smile is the standard by which all sweetness is measured. The grumble and the flattery arrive in the same breath.
The form is very tidy. Both stanzas follow the same pattern of roughly four, eight, four and ten syllables. Two quick lines are followed by a longer one, and then a long closing line carries the weight. The short lines read like a rapid exchange. "She laughs; I sigh;" is balanced on its semicolon like a see-saw, and the two verbs are less opposed than they look, since a laugh and a sigh are both just breath let out. The long last lines feel like the moment the teasing stops and the speaker says what he means. The rhymes are couplets, "sigh" with "die" and "tune" with "June." In the first stanza, "tears" and "hers" only half-rhyme, which seems fitting for a pair who are slightly out of tune with each other. That reading is my inference, but the second stanza does settle into full rhyme.
The repeated "And yet" is the hinge of both stanzas. Each time the poem seems to be heading toward grievance, it turns to devotion. Death sits at the end of both, and in a revealing way. His claim is extravagant and conditional, a lover's hyperbole of being unable to live without her. Hers is plainer and has a limit built into it, loving him "till she dies." The word "says" adds a flicker of ambiguity. It could be a tender report of her promise, or a hint that he knows she says it lightly, perhaps in the same laughing way she meets his sighs. In the second stanza her laughter has also moved on from the "tears" to the "sighs" themselves, so his sorrow has shrunk from a dramatic display to a habit she barely needs to notice.
The cuckoo gives the poem its only touch of landscape. "Once more" tells us this scene recurs every year, so the couple have a history of this teasing. The cuckoo "spoils his tune," which fits folklore that the bird's call falters around midsummer. There is a pleasing irony in a poem called "Song" that features a bird who cannot hold his song together. The cuckoo's repeated two-note call also resembles the speaker's repeated sighing, so the bird may be a sly image of the poet himself. In English folklore the cuckoo has also long been linked with infidelity. If that association is faintly present, it casts a small shadow over "she says she loves me," though the poem never makes the suspicion explicit.
Edward Thomas is generally remembered for plain-spoken poems about the English countryside, and this slight lyric shares that plainness. There is almost no ornament beyond one bird and a few ordinary verbs. What makes it worth reading is how much emotional bargaining it fits into eight short lines. The speaker is vulnerable but not pitiful, because he can laugh at himself as she does. The poem admits that love can be lopsided, with one person sighing and the other smiling, and still be completely serious. Its hyperbole and its teasing end up saying the same thing.