The junior novelization shows Judy's RESENTMENT toward Nick in a way the movie doesn't.
It makes it more obvious why she decides to leave Nick for Pawbert at the honeymoon lodge (though no less painful).
The book makes it abundantly clear that Judy wants Nick to be committed to her in terms of their partnership (even if she doesn't know how she feels about him romantically yet). She acts incredibly wounded whenever it looks like he is jilting her, and she seems to come to her ultimate breaking point when Nick fumbles the carrot pen.
In the book, Bogo points out that Judy "cares too much", while Nick doesn't seem to care at all - to which Nick replies, "Mm-yeah" (p13). This exchange sets the tone for the entire story, in which Nick has many opportunities to show Judy that he DOES actually care about her - yet continues to fumbles the ball (or, in this case, the carrot pen).
We all know he missed his chance to compliment Judy at the Zootennial gala, though he probably WANTED to. The book illustrates a private moment in which he is changing into his formal wear and thinking fondly of her:
He grinned when he noticed that Judy had brought him pawpsicle cuff links (p27).
Judy reappears in her dress, giving him the opportunity to say something. But, of course, he makes the stupid comment about her ears, leading her to PUNCH him in the stomach. Judy walks off smiling. But she may also feel a bit irritated as she proceeds into the gala. (Personal note: As a heterosexual woman, I would NOT appreciate my date mocking my appearance right after I had made an effort to look nice - ESPECIALLY if I had also just given him a present.)
This may lead Judy to fall for the false charms of Pawbert, who DOES offer her a compliment.
"I mean, you're working it. "
Embarrassed, she replied, "Well, I'm ... oh ... I, uh ... "
Nick looked over his shoulder and saw Judy laughing with Pawbert. What was she doing? (p31)
IMO, Nick is more obviously jealous in the book than in the movie. He later says, "Look, I do not like this guy" (p31). But Judy ignores him and even goes so far as to turn her ear comms off. I believe she still feels annoyed over his snarky comment. I'm not saying she was genuinely mad; however, a tiny seed of resentment may have been starting to form.
Another scene from the novel, which I think would have really benefitted the movie, involves Nick lying to Judy about having read the partnership book. Before they enter the reptile lounge, Nick tries once again to get Judy to skip town:
"It's called healthy renegotiation. Page sixty-three in our partnership book."
Judy couldn't help but be impressed as her lips formed a smile. "You actually read it." Then Judy remembered. "That book has forty-seven pages," she said, annoyed. "Come on."
"Judy, can we just---? Judy," Nick said, trying to get his partner to listen to reason. (p57)
Nick knows he screwed up here, because he immediately switches to calling her "Judy" instead of "Carrots". Nibbles later tells Jesús that Nick and Judy are "having partnership troubles" (p59), signifying Judy's mounting resentment is becoming obvious to everyone around them. (Additionally, Judy may have felt rejected by Nick's reaction to her "happy anniversary" comment on the walrus, as well as her other attempts to be sentimental throughout the book.)
After Nick fumbles the carrot pen and they both watch it smash to pieces, the novel repeatedly states that "Judy couldn't hear him". She has probably shut down emotionally at this point and needs some space from Nick. Of course, she doesn't get the chance to take any - not of her own volition, anyway - because the ZPD shows up, forcing them into action. Judy agrees to go off with Pawbert before getting shot with the tranquilizer dart. But when she wakes up later, she regrets ditching Nick, making it clear to the reader that she never actually wanted to leave her partner. She probably felt embarrassed by what she perceived to be Nick's incredible apathy toward her, but never figured out how to communicate her feelings. (Remember: Judy thinks caring about JUSTICE is the same thing as caring about HER.)
In the movie, Nick is portrayed as a slightly more sensitive leading man who doesn't really "reject" or jilt Judy. He's just kind of lazy. Still, we get to see the way he stares at her, thinks about her (i.e. by the shrine in his apartment), and talks to other mammals about her (pining for her in front of Nibbles). All the while, he constantly FORGIVES her when she screws up, puts their lives at risk, and even walks out on him. Nick is an incredibly PATIENT husband partner. He also clearly loves Judy. It's easier to be on his side when you only watch the movie.
He's not perfect, though. He still needs to display more emotional courage instead of making a concerted effort to hide his feelings. He also needs to be more proactive about going after what he wants (i.e. JUDY). I'm guessing that's why Nick was put through the wringer in Z2 - to force his emotional growth and ultimately ensure the ball is in Judy's court for the next stage of their development. He'll probably be even more of a sap in Z3 and start pursuing her explicitly, seeing as he already did a complete 180 from how he was treating her in Z1. It'll be interesting to see this ROLE REVERSAL play out - in which Nick is the one emotionally chasing Judy, and Judy is the one who must learn to stop outrunning her feelings.