I've seen lots of advice about love interests recently - probably because there are sooo many of them in YA lit. There are bad boy love interests, and boy next door love interests, and nerdy guy love interests, and sensitive love interests. There are manic pixie dream girls, and the bitchy girl turned nice, the good girl turned bad girl...
And the list goes on.
I like pretty much all love interests in moderation - although I will admit to being tired of the Patches/Edwards/semi-abusive dudes who don't deserve any girl let alone a sweet stupid overly trusting one - but I have one all-encompassing rule when it comes to writing love interests.
Nobody's going to love him/her if you don't love him/her.
That doesn't necessarily mean your LI has to be particularly likable. It's surprisingly common to love someone even if you don't like them. But if you don't get all mushy and gaga and oatmeal-brained for your character, nobody else is going to care.
Okay. So that's out of the way. Rule Numero Uno: fall in love with your love interest. But I'd like to take this rule a step further and say:
Fall in love with every character you write.
Again, this doesn't mean they all have to be likable - this simply means that you have to be invested in their fictional lives. You have to care about who they were before the story, and who they will be after. You have to love all of them as if they are your main characters; because without that love, that deep understanding of their pasts and futures and personalities, the whole story will fall flat.
Save your story. Love your characters.