Writing romantic tension

Every once in a while I'll read something where a particular element of the story really sticks out as well done. This holiday, I've been rereading Kevin Crossley-Holland's Arthur series. (The Seeing Stone, At the Crossing Places, King of the Middle March, and Crossing to Paradise) Among other things, there is lovely romantic tension. It's not overstated, it's not the major focus of the book (there are other things Crossley-Holland does well, too), but he just really, really nails it.

Sometimes I read a book where the characters just like each other because the author wants them to, but there's no chemistry. Sometimes the reason is because the MC is actually attracted to the danger the love interest represents. I'm not sure how much sticking power that has. Surely it makes for a tense plot, but will it last the lifetime of the characters? Maybe, if they face danger together, or work out the issues. But maybe not. Sometimes the reason for liking each other is brute physical attraction. I suppose that's realistic for some people, but it was never enough for me. So when I read a book where the connection--which is more than just attraction--goes deeper, it catches something in my insides. It doesn't take dates to the prom or tons of snogging or whatever. If you can convince me that the characters belong to each other, I'm sold.

So, here are five reasons why Arthur and Gatty have, IMO, such great chemistry:

1. Crossley-Holland pits Arthur and Gatty doing something right when the rest of the world isn't. They're united on the same side, against the world.

2. He uses really ordinary details and settings to do this, which makes it feel all the more grounded. (There is some magical realism in the series, but the relationships and the choices are all grounded in reality.)

3. Arthur takes some flack for stepping out to help Gatty--it’s not just that he’s doing the right thing, or that he and Gatty are on the same side. It’s that, without a fuss, he defends her, both to himself, and to others. That alone wins him some pretty undying loyalty--if I were Gatty, there is no way I could keep from feeling something for him!

4. Everyone else around them is doing wrong. But they have the two qualities I admire most—loyalty and integrity. Yes, this is a personal preference for this reader! But I think it sets them apart--you cheer for them because despite their weaknesses they are basically good people, and deserve each other in a world of injustice.

5. They understand each other’s most important feelings, despite the worlds between them.

Any other elements of romantic tension you'd add to this?