And onto the post! (/awkward)
Girl meets Boy. Girl is sure Boy is her forever and everything is going to turn out sappily every after. And then, just before a. boy asks out girl "officially" b. they're able to run away together from an evil goverment facility or c. Girl tosses aside her mortal life for Boy, she meets... Other Boy. And suddenly things have gone from the warm glow of happily ever after to a raging inferno of jealousy, passion and the knowledge that one of these Romeo's will end this book (or the book's sequel...or the third if it's a trilogy) with a broken heart.
Oh love triangles! You give stories tension (Who will she CHOOSE?!!), give characters depth (and WHY?!!), give fans rivalries and Team t-shirts.
To see what team you are, please answer this simple question. Who is Bella standing between?
A. Edward. There is no one else in the picture. (You are team Edward.)
B. Taylor Lautner's abs (happy sigh) and that creepy guy who watches her sleep (grr face). (Team Jacob!)
C. Cedric Diggory and... oh my gawd is that Shark Boy?! (You've been in a coma. Even if you don't like Twilight? You. Choose. A. Team. It's like presidential elections.)
Now, if I'm reading your book and you've done your job as a writer, by this point there's some plot going on, stakes are high and I'm making myself the One More Chapter And Then I'll Write promise...for the sixth chapter. I care about these three characters. I care about Boy 1 and Boy 2 that Girl is forced to choose between and I care about Girl and her terrible choice. I care about all of them.
And then things change. Maybe Boy 2 saves her (or she saves him!) from certain death. Maybe Boy 1 stops off at work with her favorite flowers just cuz and the hopeless romantic inside me swoons and *BAM* I've chosen a Team. Shortly after, we see Girl herself begin to lean.
And that, ladies and gentlemen, is the death rattle of a love triangle.
Once I've chosen and I'm pretty sure that she's chosen, your triangle isn't so much a triangle anymore. It's a Couple with an Awkward.
I don't want my guy to be an Awkward.
See, I usually root for the underdog (pun intended Team Jacob) and so I've fallen in love with this guy who I know will be crushed by her decision. The second she's just dragging Unchosen along because she doesn't want his feelings hurt, I'm gonna start to hate Girl a bit. But I don't want my guy left hanging. It's not fair! It's wrong! It's keeping him away from me! *ahem* The more she strings him along, the madder I get. The madder I get, the less I care about whether Girl lives or dies, or if she has sequels! Love triangles are a lot like burning a candle at both ends--sure, it gives a lovely light. Just make sure one of those sides burns out fast.