Infatuation
He was borderline obsessive, he knew that. He was well aware of his prolonged stares, of his lingering touch that had traveled from where it usually was on her lower back to her cheek while he looked at her. He knew he was in danger of crossing the line he set. He couldn't get enough of her—her scent, her smile, her laugh; the look of rage when he did something too "overprotective" for her comfort, the flash of sadness in her eyes when she went to ask Zack for an opinion only to realize he was gone, the stoic demeanor she had when something hit far too close to home. He was addicted to all things Temperance Brennan. Even the parts of her that he was rapidly learning were remnants of the long lost Joy Keenan—the infant whose whole existence revolved around people she loved whole-heartedly without ever really knowing. Even if she wasn't aware of it, she was capable of it. She loved Zack whole-heartedly, even after she learned his terrifying, gut-wrenching secret.
His infatuation with her would one day cause his demise. He was confident, however, that this would not be that day; He could love her still, from afar, with minimal risk, for another day.