Readers ship because we're left unsatisfied by the intended couple's romance. Something in the 'spark' didn't do it for us, even if we loved the book itself. Even if we wanted the characters to end up together.
'But Bethany!' you doth protest, "That can't be right! Some of the bestest books EVERZ have notorious fanfiction ships! How can that work?"
Was anyone else unsatisfied that Harry and Ginny ended up together? That Bella chose Edward?
Harry Potter, Twilight...
But, you doth rightly protest, those books are amazing!
Yes, yes they are.
And if they left some readers unsatisfied... what can I possibly do for my main love interests?
If Rowling and Meyer couldn't perfect it, how can I?
Chances are, I can't. There will always be people saying, "Screw Jacob, screw Edward, I'm TEAM JAMES!"
"VOLDEMORT IS TOTALLY BRINGING SEXY BACK! Go for it, Bellatrix!"
But maybe, JUST maybe, by learning from The Greats, we can avoid some pitfalls.
You can tell a great author because of how you can learn from their mistakes, and not just their successes.
Shall we take the cases one by one?
Harry and Ginny - It didn't bother me that they ended up together. I LOVED it. The problem was the way they ended up together. The whole process leading up to their relationship in the Half-Blood Prince was adorable and awesome. It was after that....
"Look, Ginny, there's a problem!"
"You're going to break up with me to simplify the problem, aren't you?"
"Look, problem's gone!"
"Let's get married and have kids!"
This is classic comic-book simplification. I lose about half of the respect I have for Ginny when she goes along with it. But here's the real problem... It's contradictory to the book's message. Voldemort wins when people try not to love others because it endangers them. He wins when neighbors don't talk to each other because they're paranoid. He wins when hearts are broken and he doesn't have to lift a finger to break them. Harry's not perfect, I'm not trying to say that he should be - but it isn't even addressed that Voldemort would be delighted at Harry's actions. If he bothered to care at all, that is.
Also, it's against Ginny's character that she would watch Ron and Hermione go with Harry and stay behind. I just can't see her being so passive about it.
Application: Nothing about love is simple. If the characters want to simplify their lives by not being together, address it as such. I, as a reader, have read too much about the 'I love you so I'm leaving you.' I believe that not being with the one you madly love is worse than death.
Bella and Edward - This pair, in my opinion, embodies the most common problems with love-interests. They're so obviously meant to be together, right? So what's the problem?
The love triangle book reminds me of another by Meg Cabot, which, ironically, is also called Twilight. It's The Mediator series, about a girl who can talks to ghosts. She falls in love with one, and there's this 'other guy' who's in love with her - who is also a mediator, and a bit of a bad boy. She never looks at him twice, she loves her ghost. Problem was, I was always cheering for the bad boy. It wasn't that I necessarily wanted her to end up with him... I just didn't really want her to end up with the ghost. Same way I didn't really want Bella to end up with Edward.
Well... because he's the 'perfect' love interest. And that's it. Sure, there's enough background to solidify him as a character, but his primary problems are wrapped up in the heroine. Whereas the other guy - Jacob, or Paul (the 'bad boy') is busy, you know, having his own issues. Therefore, as a reader, I get more attached to him.
Application: Give the love interest his own, unique, valid problems. He doesn't need to be a co-MC, we don't even need to know every detail about how his problems are solved. But they must be his, and not revolving about the heroine.
It occurred to me that if, when talking about our work, we refer to ANY character as 'the love interest' then there's probably a problem.
Shouldn't each character have enough of a being to be more than someones love interest?
What's the problem with your current love-interest? What famous couples left you unsatisfied?
P.S. For fun - what are your favorite ships?