If you're writing a novel, and one of your characters becomes romantically involved with another character, chances are you're writing a love triangle. I'm not just talking about young Bella choosing between two monsters who want to murder her in different ways. While it's true that more often than not, love triangles involve a choice between two people, they should also put your main character as a crossroads for reasons other than romance. Because it's called a love triangle, by definition love needs to play a role in whatever scenario you create, but more than simply asking "Whom do I want to be with?," a good love triangle should force your main character to ask, "Who do I want to be?"
(Note: This is more commonly written with women having to choose between two men. Presumably, this is because it's more socially acceptable to portray women as "conflicted" or, alternatively, "in control of their own destinies" Sadly, men are rarely the chooser in a love triangle because they are still given the unfortunate stereotypes of not really caring about relationships or needing to make a choice. Men are never praised for being "independent," or even labeled as such, because it's implied by society that they already are.)
If there is romance in your novel, you will likely need to employ a love triangle of sorts to add conflict to your relationship, or would-be relationship. The typical set-up is this: There's the good guy and then there's the bad guy... but is he really that bad, or just misunderstood and Mr. Darcy-esque? What's a girl to do??
In most romantic love triangles, the choice is almost always between two guys - one is sickeningly perfect and the other is sort of a dick, and both are hot. The choice here should be simple, but in the best love triangles nothing is as it seems.
Take my favorite incestuous love triangle at the moment - Stefan/Elena/Damon from The Vampire Diaries. With vampires, you can usually tell which one the "good guy" is by their willingness to kill animals instead of humans. This is apparently more acceptable. Stefan only feeds on animals. And he loves Elena and is nice to people and isn't so damn sarcastic. Hence, "good guy." Then there's Damon, the shirtless wonder who thinks nothing of (literally) tearing someone's heart out just for fun. He also loves Elena, but in a creepy "you will be mine" way. Hence "bad guy."
But if things were that cut and dry, the show wouldn't be nearly as successful and their love triangle wouldn't be remotely fun to watch. All signs would point to Stefan. But then we learn that Damon didn't actually want to be a vampire and his lack of emotion actually stems from bitterness toward Stefan, who turned him in the first place. We also get glimpses of real feelings that Damon has for Elena, as poorly executed as they may be sometimes. Then there's Stefan, who also becomes more than just "the good guy." We learn he's a recovering addict who, when he falls off the wagon, falls murderously hard. Plus his history of having strictly platonic, meaningful friendships with other women adds some swoon-worthy depth.
By creating a balance between the characters and making them more ambiguous than their designated roles suggest, Elena has an actual choice on her hands. Unlike another one of my favorite teen-centric shows, Veronica Mars. Season 2's Duncan/Veronica/Logan love triangle was a perfect example of how not to write a love triangle.
Enter Logan Echolls: rich, privileged, snarky, huge temper, and prone to screwing up basically everything in his life. But then there's the boyish smile, the charm, the humor, the fact that he wants to change, and his overwhelming affection for Veronica.
Now enter Duncan Kane: Rich, popular, and attractive in a mundane way. He's also puppy dog-esque, "good" in the sense that he doesn't beat up everybody like Logan, passive in that he never tells Logan to stop, and his affection for Veronica seems to be based more on "hey you're not my sister after all!" than actual attraction, appreciation, or even mutual interests.
Basically there is no contest.
If you're writing a male character having to choose between two women (congratulations!), then this "good vs. evil" dilemma usually comes in the ever-flattering form of "vixen vs. nun." You have the hooker-with-a-heart-of-gold character, someone who's from the wrong side of the tracks who would clearly never, ever be good enough to win the love of any man. She does lewd things like not use the correct utensil at dinner and enjoys sports. Then there's the well-to-do, maybe a little rigid, but certainly very beautiful "good girl." The one who comes form the good family, has a good job, and looks good on paper.
Again, when are things ever that easy? John Cusack's character, Lane, in Better Off Dead loves pretty, blond, popular Beth to the point where he'd rather die than live without her. Then some French girl who wears trenchcoats and can fix cars comes along and... conflict ensues. Monique might not be as hot as Beth, and she's certainly not the typical girl Lane saw himself being with, but the two have much more in common than he did with Beth. This doesn't make Beth purely evil though. She seems shallow by comparison, but when we get flashbacks of their relationship, we see they actually shared the same sense of humor and had fun together. She wasn't all bad; she just wasn't for him.
When your main character is faced with a choice, make sure he or she has a hard decision ahead of them. If one person was purely good, and the other is purely a disaster, then why are we reading? The reader needs to know why both options are viable. What does the main character see in either of them? Why should we, in turn, like both of them too?
Also consider the fact that in real life no one is ever so one-dimensional. Real people, even good people, have issues. The "bad" people also usually have more under the surface than just anger. Yes there is a Mr. Darcy fantasy involved that all jerks are just good guys at heart. Call me an optimist (or a masochist), but I believe that most people are good people. People just have different ways of showing it, and some (yes) have an inability to show it. But, falling in love with someone isn't typically easy, and there is usually more than one side to look at before choosing your soul mate. A good love triangle explores all of those sides.
Love triangles are important in fiction for non-romantic goals too. As I mentioned above, there is a question of who the main character becomes based on his or her choice. That choice doesn't necessarily need to have a purely romantic outcome though. If you're writing a novel with romance in it, but aren't planning on making another man or woman the source of conflict, you still need to add the third element. If your main character chooses against love, what are they choosing instead?
If your main character's goal in life is to become a ballet dancer or rock star or move to Europe, and they spend most of the novel trying to reach that goal, will they throw it away for the love interest they meet along the way? Think of what your character needs and how the events of your novel brought them to this decision. Sometimes love wins, sometimes the life that can't include love wins. Either way, there is sacrifice. But what is important to remember as a writer is to present both options equally so that no matter what your character chooses, the reader understands their decision, and chooses right along with them.