Suzanne Brockmann said this quote

As Ian looked at her, he felt something in his chest slip and shift. The pressure came with a blood-tingling rush of triumph and satisfaction, pride and a deeply burning sense of possessiveness. His inner caveman warrior had been awakened and wanted to rush around the room, peeing into the corners, marking it—and her—as his own, while shouting Mine! and randomly smashing things for emphasis.But he knew that was he was feeling was the equivalent of emotional and hormonal indigestion. He hadn’t done this in a long time. And he particularly hadn’t done it with a woman he liked as much as this one. In fact, he’d never had sex with anyone that he genuinely liked as much as he liked Phoebe.