Writing a Strong Love Interest

You probably didn't think I'd write about this subject, because I don't read romance. But I read books with romantic subplots, and I've developed a sort of preference for the love interests I like, and I'm going to share my opinions here. They're just my opinions, just like everything on this blog. Don't take it as gospel, just keep it in mind while you're writing your love interest.

1.  Make them likeable. I haven't read a terrible love interest, but what I've heard is that a lot of writers create love interests that start out as total jerks, for the sake of being "fixed" by the protagonist. This is ridiculous; if your MC is falling for a total jerk, she doesn't have very good judgment. In real life, a boyfriend like that will likely require a restraining order. 
2. Make them compatible with the protagonist. People do say "Opposites attract," but there should be something they have in common. Maybe they're both huge geeks, or are both haunted by similarly dark pasts. 
3. Make them flawed. A flawed love interest is much more interesting than the confident, muscular, Romeo or the beautiful, gentle Juliet. And if your love interest is perfect, why would he/she want to date your flawed protagonist? (I'm assuming you're making him/her flawed, because flaws are an important part of any character.) 
4. Give them identity and a backstory. Love interests should be real people, with history and personality. If they're just a one-dimensional cardboard cutout, your readers won't identify with them. And if they don't identify with a character, they won't care about them. Have your love interest be as real as your MC.
5. Let your characters fall in love slowly. Nobody wants to read about people who fall in love by the first page. They need to learn about each other and get to know each other before they can really fall in love. 

Remember, this is applicable to all genres with an element of romance. This applies to your science-fiction, your historical fiction, anything. And this doesn't just apply to your protagonist; anybody in your story can fall in love with anybody else.

I hope this helps! Happy writing!