The speaker stands by while a boy talks about a girl, and the poem is built around what goes unsaid. We never see his words, only the speaker's view of him: his eyes growing bigger, his blush at the mere thought of her. These small details show how much he cares, and the speaker notices every one of them, which suggests she has been paying close attention to him for a while.
The title, "sarah," and the line "it would never be me" tell us the speaker is not the one he wants. Her question, "how could i," sounds less like a choice and more like being stuck. Speaking up would mean either hiding her hurt or admitting it.
The mood is quiet and resigned. Short lines, no punctuation and lowercase letters make the poem feel hushed, as if the speaker is shrinking back. The last line lands flatly, like a fact she has already accepted, and that calm makes it sadder.