Fans are always looking for clues as to which ex-boyfriend her songs reference, especially after a string of breakups. Luckily, this period of Swift's career is behind her, as she is currently in a relationship with Joe Alwyn.
This song is about pining for someone who "everybody else wants." The title comes from a legend of sailors who were told that a ship with sirens singing drowned men's ships. The narrator of the song relates this story to her own daydream about her love interest: they're beautiful and their hair is perfect.
The narrator calls her dreamer a “golden boy” and goes on to describe them as being the kind of guy who would make anyone's life "shine." She imagines this dreamy guy being at her house, dinner parties with her, and on a beach getaway together.
But the end of the song comes as a shocker, as she realizes that this isn't a real gold rush. It's a tunnel vision of what could be, but it can never be. She doesn't want to live in the dream for too long, because it's not worth the risk.
The song also features some sophisticated satire, which is part of a shift in Swift's style as she matures as an artist. She's not only interested in constructing narratives anymore, but she also wants to reframe them. This is what makes her songs so compelling.