Continuity: Anytime before Season Two's 22nd episode, Casino Night.
It's kind of funny because it doesn't make sense. Granted, according to Physics, opposites attract. But they're not opposites - they're almost the same, so the rule doesn't really apply.
The irresistible, charming, magnetic attraction between them, however, becomes stronger by the day, even when it wasn't even supposed to be there in the first place.
Because they're friends - best friends - and ruining such a perfect, cheerful relationship wouldn't be okay. And because she's engaged, which means that a) he has no right over her, and b) if she happened to have feelings for him, she wouldn't be able to act on them anyways.
But the attraction remains. They don't understand it, sometimes they curse it, sometimes they wonder why it exists - maybe the fact that they can't be together generates it out of pure thrill and excitement; yeah, perhaps prohibition and physical attraction are directly proportional – and mostly they just bear with it.
Neither of them is ready to face what's beneath all that attraction.
Neither of them is willing to see past that force that pulls them closer and closer and closer as if it (whatever it is) wanted them to become one, a whole.
It's getting harder by the second for them to pretend they don't want that as well.