Ten reasons Zane Donovan knows he is in love with Jo Lupo:
When he closes his eyes, he sees only her. The way she looks in her all-black GD uniform and the way she looks out of it – "The Enforcer" and the lover. The way her frowns reach her eyes and the way her smiles reach his heart - the old Jo and the new Jo.
When she comes out on top, he doesn't mind. He is proud of her for never backing down in a fight, physical or verbal, and he likes the challenges she presents. No matter how hard he trains, how much he works out, she can still physically overpower him. And she does, every single time. They also bicker from time to time, and he rarely wins. Neither type of loss leaves him mad or feeling less like a man. They make him want to work toward becoming a better man.
When she is nearby, his heart races. He never has to see or hear her walk into a room because when his heart races, he knows she is nearby.
When his eyes are open, his thoughts drift to her. He often finds himself distracted from the task at hand (any task at hand). It isn't so bad during the mundane events of his days, eating at Café Diem or driving home from work. It can be, however, dangerous when he is at GD handling highly dangerous devices.
When she touches him, his breath catches in his throat. The feel of her fingertips sends shivers down his spine, and the sensation of her body pressed against his as they sleep makes him feel secure and happy.
When he thinks about his life, he focuses on the future. He used to dwell on the past. He worked only for himself - and whatever establishment was holding him hostage. Now, she fuels the fire within him, and he works to build a better life for the two of them, but most of all for her. He wants to experience the world with her, carry her over the threshold of their first home, see her belly grow with their children, make memories with her, grow old with her.
When Fargo told him about the other timeline and Other Zane, he didn't flinch. In fact, he wasn't all that surprised. He was surprised when Jo ran into the sheriff's office saying she'd marry him. He was surprised when she gave him his grandmother's ring back even though he didn't remember ever giving it to her. He was never surprised to discover that their relationship could be much more than the professional label of officer and criminal. Besides, her fancy high-tech handcuffs have its perks.
When other men flirt with her or undress her with their eyes, he gets jealous. (The kind of jealous that makes him want to rip out their tongues and eyes, not the kind of jealous he used to get when he couldn't meet the guys for drinks after work.) No one has those rights but him. He thinks she might get mad if he referred to her as his girl, but he knows she is.
When Eureka is in danger, as it commonly is, his first instinct is to protect her. He panics if he doesn't know where she is or if she is all right. The pressure he experiences in these situations comes directly from his fear – his greatest fear - that something bad will happen to her. He would sacrifice himself to save her. He would even endure the matrix for the rest of his life if it meant she would remain safe and sound.
When he thought he'd lost her, he felt a part of himself die.