You know that cliché in musical theatre when two characters who have just met fall in love and sing a gorgeous love duet? If your Valentine's Day included something like that, here are three reasons why it worked.
(Note: love duets only, so for example “Now I've Seen You” from Honk and “What Do I Need With Love” from Thoroughly Modern Millie aren't included; it's one thing when a character falls in love with someone but quite another thing when it's immediately reciprocated.
Also, I'm not including operetta. Once upon a time it was just expected that the male and female leads would walk onstage, meet, and instantly fall in love, but since “If I Loved You,” musical theatre has needed to justify its love songs more 'realistically.'
Some wonderful musical theatre love duets are sung early in the show but by characters who have known each other for a while, like “Pineapple” from Cabaret or “You're the Top” from Anything Goes. I haven't included them here because the justification is pretty clear. For clarity I've included a count on how long these characters have known each other. None have known each other longer than a week.)
Youth
Young people fall in love quickly and easily in both musicals and the real world. How many kindergarten marriages are out there? These characters are a bit older than that, but most are teens and none are out of their 20's. Except maybe Irene Molloy and Cornelius Hackle, but Cornelius is a man-child and Irene is desperate and sorta gets swept away by it all.
Sometimes it’s happily ever after, like “A Heart Full of Love” (one afternoon), “It Only Takes a Moment” (about six hours). Sometimes it makes life really difficult for a while but everything is great in the end, like “Say It Somehow” (maybe a week?) and “Lovely” (first time they've met). And sometimes there's an actual blood bath, like “Tonight” (maybe three hours?) and “Kiss Me” (second time they've met). #thankssondheim
The End of the World
Rare but surprisingly mature: a bit heavy for most musical comedy! These characters exist in a world where death is all around them so it's easy to get carried away. They don't sing about “moon and June,” they sing about more everyday kinds of things, like living together and doing dishes.
Everyone's dying of AIDS as Collins and Angel sing “I'll Cover You” (less than a day) while it's World War II that'll get everyone in “Some Other Time” (20 hours at most).
Convenient Excuse
Yes, the characters are falling in love, but they're really singing about something else entirely. Sometimes it's a convenient excuse to hide their true feelings, sometimes they fall in love because of the song. But these are love songs that have nothing to do with, all to do with love (pun).
“Falling Slowly” (one afternoon) starts as just a performance. “If I Loved You” (first real conversation) has that beautiful IF to provide some distance. “I'll Know” (first time they've had a conversation) both has the distance of “If I Loved You” and is a con – Sky is wooing Sarah for a bet. And “Baptize Me” (less than a week) is just about a baptism, right? Nothing else going on here!