Let's talk Irrelevant Characters.
I've already mentioned... repeatedly... that I hate love interests who exist just for the sake of being a love interest. But what really is the issue with this?
In my opinion, the worst thing you can do for your novel is create a check-list of things that you need. For example, if you're writing a Fantasy novel, you might create a check-list like this:
- Young hero, prophesied to be the "Chosen One"
- Wise old wizard
- Spunky heroine, raised herself in the woods
- Quest for MacGuffin
Why is this so harmful to your book? You're not following the story. You're treating writing a novel like buying a furniture kit from Ikea. If you can figure out how to fit Fig. B to Fig. C, then you'll get the perfect novel. And we all know it just doesn't work that way.
Throwing in a love interest comes with the same mentality. You think, because Romance is one of the biggest genres, and everyone loves a good ship, that you need a romance in your story. But these love stories tend to lack chemistry and true interest, because the Love Interest has no real substance. They aren't thoroughly developed characters. They don't actually matter. They serve as motivation for the character as much as any MacGuffin.
Oh, and then, of course, you do something terrible to the love interest -- even kill them -- just to emotionally scar your hero. Not only is this cliché, it's just terrible. Is that really the only thing you can think of to motivate your hero? Is that really all this character is good for? The death of a character should make the reader feel like they're losing something. Like this story can't possibly go on with that character missing. That's how it feels when we lose someone in real life. How can the world keep moving with that person gone?
Look, I'm not sure I'm making this point very well. All I'm trying to say is that you need to follow your story. Trust it. I'm not saying don't have a Love Interest. Love stories are wonderful, and love is a great motivator. But sometimes, throwing in a ship just gets in the way. Let's be honest, so many teen novels have brilliant situations and great characters, but the love story weighs it down and makes half the book gag-worthy. Does every teenage rebellion leader really need a boyfriend? (For the record, I haven't read Hunger Games or Divergent, so don't think I'm talking about any of these teen novels in particular. I'm just judging the genre because I'm a stereotypical snot like that.)
Now, it's just as bad for your story if you make a list of things to AVOID because they've been done before. To never use prophecies, wise old wizards, or plucky heroines because they've been done to death. If your story requires a Love Story, killing a Love Interest, or even a Love Triangle (a topic for another day), then please add it.
What it comes down to is analyzing your motivation. Are you adding in a Love Interest because you're "supposed to"? Because it might make your book more popular? Or are you adding them because they are an important character? Because there would be a hole in your story -- entire plot points that didn't work -- if they were missing? And this doesn't just go for Love Interests. This goes for sidekicks and mean girls and plucky comic reliefs. Everything in your story should have to FIGHT to be there. Does it add interest? does it carry the story forward?
When choosing what to add to your novel, make sure you're listening to the story itself rather than what is popular. That's the only way you're going to get quality.