Perhaps some people Google this question in search of an explanation for the sudden appearance of birds – before an electrical storm, say, or on the second day of Christmas. Most, though, are looking for Burt Bacharach’s famous song, using its first line rather than its clumsy title: (They Long to Be) Close to You. The song was a huge hit for the Carpenters, and the question it opens with – “Why do birds suddenly appear / Every time you are near?” – is the one I’m going to answer.

To state the obvious, it’s a rhetorical question. As the singer well knows, it’s because she is in the sweaty yet vice-like grip of infatuation. This state of mind is the point of the song, which doesn’t tell any kind of story. Who she is, who he is, where they met (if they’ve even met), we don’t know. (And before you ask: no, she’s not a stalker – as the gentle sway of the tune makes plain, there is nothing sinister about this crush.)

Richard Chamberlain was the first to record the song, in 1963, at which point the last verse said “all the boys in town / Follow you around”. For female vocalists these boys were changed to girls, so there’s no sexism in the use of the diminutive: this overpowering attraction is a unisex one. The “hair of gold” and “eyes of blue” in verse three, on the other hand, were never altered, which might help explain why Dionne Warwick’s version never caught on in the way the Carpenters’ did. Did an African-American soul star swooning over a blond-haired, blue-eyed boy not have quite the same ring in 1964 as the all-American, white Karen Carpenter did when the single shot to No 1 six years later?

My husband’s job is to write about pop music, and while I don’t I’m used to talking about it. But my breezy offer to write about this song, made after my choir sang it, felt over-hasty as soon as I’d made it. I’m far from expert on the prolific, brilliant songwriting partnership of Bacharach and lyricist Hal David (they wrote Raindrops Keep Fallin’ on My Head, Walk on By and I Say a Little Prayer among other standards).

Not everyone at choir loved the song. Some of David’s words – his blond-haired, blue-eyed dreamboat; his angels sprinkling moondust and starlight – feel as though they’ve walked out of a book of romantic cliches. But you can’t argue with that first line – so sweet and yet so serious. Why does the world look different when we fall in love? Why do those we lust after appear enchanted? Are these things really happening, or are our minds playing tricks on us? The singer is curious, but aren’t we all?

I could have attempted a scientific explanation all about hormones and chemicals – how testosterone, oestrogen, oxytocin and serotonin are part of the story of sexual attraction. Do birds suddenly appear when we are fertile?

Or I could have written a psychoanalytic one, about how the feeling of falling in love reawakens some of our earliest experiences of gratitude. Do birds suddenly appear because our capacity for appreciation has been fed by secret springs of emotion?

Both answers might be true, but neither says anything about the tune – without which the words would be nothing. So I phoned Lis Stewart, the music teacher who runs my choir, to ask her what it is about the melody that has made it stick around for half a century. She said the trick is the way the rinky-tink introduction, so familiar and stuck in one place, suddenly stops – so the three notes that follow (Why do birds … ) come as a surprise. Also, the intervals between these notes are unusual: “It’s not moving step by step, it’s kind of leaping, it’s the rising intervals that always sound hopeful.”

There’s no verse-chorus structure either, which makes the whole thing more fluid. It also means that while the chorus is usually the catchy bit of a song, here it isn’t. Instead, of course, we remember that upwardly lilting question: “Why do birds suddenly appear / Every time you are near?”

It’s because she fancies him.

• Susanna Rustin is a Guardian writer and editor