Basic Trope: Someone loves someone else, but their love isn't returned - until they enter into another relationship.
Straight: Bob is in love with Alice, but she doesn't return his feelings. When he gets into a relationship with Carol, she falls for him.
Exaggerated: Every part of the Love Dodecahedron flips when someone enters a relationship.
Downplayed: Alice wasn't entirely oblivious before, but once Bob and Carol become official she realises she's jealous.
Alice didn't realize she loved Bob until he wasn't on the table anymore.
Alice liked Bob as a friend because he had a great personality but saw him as physically unattractive. But as it turns out Bob cleans up quite nicely and now she's all over him.
Alice is the type of person who wants what she can't have.
When Bob was pursuing Alice, she found him to be annoying, clingy, and possibly only being kind to her in hopes that she'd feel obligated to love him back. But once he starts going steady with Carol, he's more laid back, doesn't stalk Alice's every move, and his friendship feels more genuine. As a result, Alice starts liking him after all.
When Bob realized Alice was a lost cause, he undergoes a personality shift, becoming someone Alice likes.
It's not until Bob's in a relationship with Carol that he realizes he loves Alice.
Alice starts hating Bob when he doesn't hate her anymore and now hates someone else.
Alice was Bob's Unknown Rival. But Bob started to take their rivalry seriously around the time she got a new rival, making him her Unknown Rival.
Subverted: Bob gets into a relationship with Carol. Alice is glad because that means he'll stop trying to pursue her.
Double Subverted: But after a while, she realizes she secretly enjoyed being pursued...
Parodied: Alice openly and repeatedly mentions her vitriolic hatred for Bob - until he gets into a relationship, when those feelings are replaced with slavish devotion.
Averted: Alice just plain isn't interested in Bob, before or after his new relationship, and is happy (and/or relieved) that he finally found someone.
Enforced: "OK, we've dragged out the Unresolved Sexual Tension between these two enough, but how can...wait, I've got it! Let's have him get into a relationship with someone else!"
Lampshaded: "I know it's ridiculous, but now I can't have him... he's totally hot!"
Invoked: Operation Jealousy: Bob starts hanging around with Carol in the hopes of getting Alice to see what she's missing.
Defied: Alice is well aware that Bob is only dating Carol to get back at her, and is disgusted that he'd use Carol that way, making Alice hate him even more.
Deconstructed: Bob realizes that Alice only started to like him once he was in a relationship with Carol, despite him persuing her before. If it weren't for that, Alice might have never wanted to be with him. He is disgusted by this and cuts off all ties with Alice, who laments on losing a great guy.