"I love you." To writers everywhere because the writers of "Heroes" has to learn this. "I love you" is a powerfully earnest thing to say to someone. You can tell someone you're attracted to them or you like being with them but people generally don't hand out "I love you's" until there's been some courtship and an emotional bond is organically formed.
Or it can be a matter of instantaneous personal chemistry, where two people are attracted to each other immediately. I could buy that, too, but the actors have to sell it, or maybe they just have to have it inherently, to enough of a degree that the audience can see it, so that it's more a question of the right casting than good acting.
Instant-attraction relationships can easily run right off a cliff. They can be catastrophic and unhealthy (and here I'm thinking specifically of a new relationship just introduced on
Heroes, of a very, err, electric variety) but who cares? That's just more drama and fun for the audience, even if not for the characters. The important thing is that we have to buy the idea that it would happen at all.