For some reason, the heroes and the heroines wind up dancing to some slow romantic song that requires waltzing or some other dance typified by close body contact.
Hey, that's ok, nothing special except she's starting to blush, and he's getting all nervous. Could it be? Are they in love?
This often pops up during high school dances and fancy parties, the kind with hooped skirts and masks.
For some reason, waltzing with a friend, or friend-of-a-friend, always winds up with one or both participants acquiring (or realizing) feelings of love towards the other, as though it were some kind of movement-driven truth/love spell.
Often prefaced by an observation that She Cleans Up Nicely, and frequently occurs during a High School Dance, though it is by no means exclusive to teenagers.
When adding examples, keep in mind that this is about the dance being the first indication of romance between the dancers, not just a romantic dance between two people who are in love (or in denial).