Love
Stein introduces different forms of love as a major theme of the novel. The first relationship to demonstrate love is that between Denny and Enzo. Denny and Enzo experience "love at first sight" when Denny lifts little puppy Enzo out of a furry pile of puppies in Spangle. There is no explanation as to why Denny picks Enzo; there is simply a magnetism between them that we understand as fate, evidence that Denny and Enzo are meant to be together. Their love grows into a trusted companionship where they rely on each other for unconditional support. One of the pros of Enzo not being able to talk back to Denny (despite how badly Enzo wants to be able to speak) is that Denny knows he can say anything to Enzo without receiving a judgemental response.
When Denny meets Eve, the novel shows the reader romantic love. Denny and Eve's relationship progresses quickly, and Enzo finds himself competing with Eve for Denny's attention. By putting Enzo and Eve at odds, the novel demonstrates how different forms of love can distract a person from their respective relationships in life. But this tension is resolved when Eve becomes pregnant with Zoë, and Enzo immediately softens toward her and accepts her into the family.
The novel demonstrates familial love between Denny, Zoë, Eve, and Enzo. Both Denny and Eve ask Enzo to protect the other members of their family, which demonstrates the universality of a parent's and spouse's instinct to protect their family members. Even The Twins, Maxwell and Eve, understand the instinct to want the best for their family, even if their goals and methods are arrogant and perverse.
Another type of love the novel portrays is fleeting infatuation, as demonstrated when Annika tells Denny she loves him right after attempting to have sex with him while he slept. Stein shows this type of love to be impulsive, and Annika's age reinforces the idea that infatuation is an immature type of love.
Illness
The theme of illness connects the life experiences of Eve and Enzo, which provides Enzo with a first-hand perspective of what it feels like for one's body to fail or betray them. Illness plays into the theme of losing control in unexpected circumstances, which is a major element of the main extended metaphor of the novel, which is racing. When Denny philosophizes about racing, he emphasizes a good racer's ability to react to chaos as it happens. In Eve's case, she chooses to ignore her symptoms and avoids any contact with doctors or medical care because of her fear of a diagnosis.
Eve's approach contradicts Denny's, so at first, Enzo regards her choice not to see a doctor as a big mistake. But when Enzo finds out he has hip dysplasia, he suddenly understands why Eve avoided a diagnosis. Enzo feels labelled by his diagnosis and that now that his pain has a name, it will only grow stronger and more debilitating. He is sure that his hip dysplasia will ultimately kill him if something else doesn't first.
Communication
The novel begins and ends with Enzo's struggle to communicate his thoughts and feelings. Since he thinks like a human and understands language, he has internal reactions to the conversations around him, but his reactions remain bottled up inside of him. Enzo's inability to communicate underscores how the human beings around him fail to communicate with each other, despite the fact that the humans do posses the ability to speak. An example of this is when Zoë refuses to eat her chicken nuggets, causing Eve to lose her temper and slash her hand open while trying to angrily open a frozen pack of hotdogs. Enzo discovers later in the evening that the nuggets were spoiled, and Zoë simply lacked the vocabulary to express her reason for refusing to eat the nuggets. Later in the novel when Denny either fails to notice or fails to speak out against Annika's flirtations, he allows for the terrible situation which leads to long criminal and custody trials. If Denny had simply communicated clear boundaries between himself and Annika, he could have avoided the years of hardship he endured as a result of the false accusations. The fact that Enzo notices Annika's flirtations from the beginning emphasizes Denny's culpability in his failure to communicate boundaries.
Gesture
Closely related to the theme of communication, the theme of gesturing arises from Enzo's inability to communicate. A gesture is a form of communication that relies on actions instead of words, and it is not limited to dogs or animals who cannot speak. Sometimes, gestures are even more powerful than words, because we can project multilayered symbolic significance onto them. An example of Enzo gesturing is when he rips the custody settlement out of Denny's hands right before he signs them and then takes them outside and urinates on them. The clarity of Enzo's gesture convinces Denny not to sign the documents. A gesture can be a strong indication of character. At the end of the novel when Maxwell and Trish drop the custody suit, they ask for an additional 48 hours with Zoë to pack her things and spend an extra couple of days with her before she moves to Italy. Denny could easily deny them the extra time, but in a gesture of goodwill, even after all they put him through, he grants their wish. Another example of humans gesturing in the novel occurs when Denny's parents show up after years of absence and offer him money from mortgaging their farm. Since so many years had passed between them without communication, Denny's dad hopes that the gesture of offering him money will help mend their relationship.
Loss of Control
Loss of control plays into the extended metaphor of racing. How a person reacts to a tailspin or an instance of chaos or vulnerability in their lives determines their trajectory. For Denny, a marker of losing control is drinking liquor. When Enzo rips up Zoë's stuffed animals, Denny reacts by smacking him in the muzzle. Denny had never before hit Enzo. Shortly after the smack, Denny cools down with a liquor drink; so the reader is meant to associate out-of-control Denny with liquor. The second time Denny resorts to liquor is immediately after he is bailed out for sexual assault charges. So many different things go wrong for him at once, namely Eve's death, a custody suit from her parents, sexually assault allegations, and financial ruin, that his situation seems hopeless. But Enzo refuses to let Denny capitulate, and through patience and perseverance, he is able to regain control of his life.
Evolution
Enzo thinks a lot about evolution and how a dog's biology relates to a human's. This leads to his discussion of "dew claws," which he believes to be a pre-emerging thumb in canine biology, thus "proving" his theory that dogs are closely related to humans. Enzo's jealousy of monkeys also relates to the theme of evolution; because monkeys are thought to be the closest relation to homo sapiens in the animal kingdom, Enzo decides that he doesn't like them at all. Several times throughout the novel Enzo laments his lack of thumbs, like when Eve leaves him locked in the house for days. If only he had thumbs, he would be able to turn the doorknob and let himself out. Enzo's discussion of evolution culminates in a flashback of when he was a puppy in Spangle and the vet cut off his dew claws without anesthesia.
Reincarnation
From the moment Enzo sees the documentary about dogs in Mongolia at the beginning of the novel, reincarnation occupies much of his thoughts. Reincarnation gives Enzo hope that someday he will be able to communicate with language and fully express himself to humans. It also gives Enzo the tools to explain why certain senseless deaths occur in the world. For example, when Enzo explains the death of Ayrton Senna, he concludes the section by saying that Senna's body had served its purpose in the world and that somewhere, shortly after Senna's death, a child received the champion soul of a racer. The concept of rebirth allows Enzo to imagine a possible future where he can race cars like Denny; the epilogue implicitly fulfills Enzo's imagination by suggesting that he was reborn as an Italian boy with a passion for race car driving.
The religious concept of reincarnation opens the novel up to engaging with Buddhism, Hinduism, and other cultures who recognize rebirth as a part of life. It also opens Enzo up to engaging with other animals who are thought to have supernatural associations, like the crows that fly around their house in Central Seattle. In some cultures, crows, like dogs in Mongolia, are regarded as the second body before a soul becomes human.
