A critical element to great chemistry is respect. Your hero and heroine should see each other as equals, and not out of some politically correct we-all-have-the-spark-of-divinity worldview, but because they forcibly strike each other as so.
A perfect example below, from the Loretta Chase classic Lord of Scoundrels:
“Perhaps I had better demonstrate how the thing operates,” said Dain, yanking her attention to him.
In his low voice, Jessica recognized the too innocent tones that inevitably preceded a male’s typically idiotic idea of a joke. She could have explained that, not having been born yesterday, she knew very well how the timepiece operated. But the glint in his black eyes told her he was mightily amused, and she didn’t want to spoil his fun. Yet.
“How kind,” she murmured.
“When you turn this knob,” he said, demonstrating, “as you see, her skirts divide and there, between her legs, is a-” He pretended to look more closely. “Good heavens, how shocking. I do believe there’s a fellow kneeling there.” He held the watch closer to her face.
“I’m not shortsighted, my lord,” she said, taking the watch from him. “You are quite right. It is a fellow-her lover apparently, for he seems to be performing a lover’s service for her.”
She opened her reticule, took out a small magnifying glass, and subjected the watch to very narrow study, all the while aware that she was undergoing a similar scrutiny.
“A bit of enamel has worn off the gentleman’s wig and there is a minute scratch on the left side of the lady’s skirt,” she said. “Apart from that, I would say the watch is in excellent condition, considering its age,though I strongly doubt it will keep precise time. It is not a Breguet, after all.”
She put away the magnifying glass and looked up to meet his heavy-lidded gaze. “What do you think Champtois will ask for it?”
“You want to buy it, Miss Trent?” he asked. “I strongly doubt your elders will approve of such a purchase. Or have English notions of propriety undergone a revolution while I’ve been away?”
“Oh, it isn’t for me,” she said. “It’s for my grandmother.”
She had to give him credit. He never turned a hair.
“Ah, well, then,” he said. “That’s different.”
See what I mean? With her poise and her presence of mind, Jessica Trent forcibly strikes Lord Dain as an equal–or at least as someone he could not easily dismiss. Tremendous chemistry in that book and little wonder.
Another excellent example would be the movie Mr. & Mrs. Smith–and if you haven’t seen it, please do so at your earliest convenience. The whole of Mr. & Mrs. Smith is a metaphor for the modern marriage. At the beginning of the movie the titular couple have fallen into a complete familiarity-breeds-contempt rut. Then, as they discover each other’s secret identity, things heat up–they have to consider the person they thought they knew in a whole different light. And during one of the movie’s pivotal scenes, when they are fighting mano-a-mano, that mutual respect is literally pounded, kicked, and whacked into them. And the old fire roars back to life because there is now something much stronger to feed it.