Okay, before I get started let me get on my soapbox and say many a good writer stumbles by confusing relationship building with sex. They are not interchangeable in fiction any more so than in life. And this writer is the reason romance is marked as smutty. Also, I promise not to read any romance writer using sex to replace relationship building. Like ever. Of course it's easier--that's true to life. It just usually doesn't lead to an HEA. (It doesn't in life--so HEAs like this in fiction don't ring true to me). And I have to roll my eyes that this is "romance."
::Steps off soapbox.:: So how do you build a relationship then?
The same way you do in life. You stumble and spill coffee on your date then have to process that he didn't freak out. Only set you straight and asked, "Are you okay?" You go to a movie. Or you play vampire baseball and have to flee a tracker together. You go see an orchestra in Portland on the premise a guy you go to school with won tickets he has no use for in a radio show and then learn he saved pizza tips for two weeks to buy those tickets and wants to know what you think about it. Characters talk. They make gestures--they act. They re-act. They find things they like--love about each other, often times things they find lacking in themselves. And I'm not talking about insta-love here either. That usually doesn't work so well, although if it's played out right love at first sight can work. But often there is some kind of instant respect or admiration. And sometimes even that doesn't exist. Then you need attraction. There has to be a spark of something to ignite the fire and unleash the story.
The harder part: once you've established this love it has to be challenged--threatened--pushed to a possible breaking point. And just like in life this sucks. How do you do this? A character makes a bad decision the other person is absolutely not going to accept, or they're hiding something in their past, or the characters realize they have a difference in some kind of core value. Let your characters screw up. Push further. Instead of just coming clean about it, they lie--cover it up. Now not only are you a loser but you lied to me too. Push it to a breaking point. Make them fight for each other.
* Unrelated-- but I'm teaching a workshop on blurb writing in June. I used this process to query with a 50% full request rate before signing with Kathleen Rushall of Marsal Lyons Literary Agency. And I use it for my self published blurbs.*