I am convinced that one of the main things that drew Mark so close to Sprite was their parallel paths: Mark had recently experienced and was still plagued by a life-threatening heart condition when Sprite entered his and his family's life. Unbeknownst to the Levins, Sprite was much older than they had thought and began manifesting major health problems almost from day one.
The Levins' heart wrenching realization that Sprite wouldn't have much more time on Earth, I believe, quickened their feelings for him and made them appreciate him and their other dog, Pepsi, that much more. This, coupled with Mark's own problems, caused Mark to deal more with his own mortality and to rearrange his own priorities, placing his family and friends above all other things. "No time on Earth is long enough to share with those we love or to prepare our hearts for good-bye." And, "Career and financial goals are important, material acquisitions are necessary, but taking stock in life's little pleasures is the most satisfying experience of all."
Sprite touched Mark in a way he'd never been touched before, and taught him more about life they he could adequately express. "Sprite touched my heart and opened my soul. I would swear he was an angel."
Mark describes how Sprite's handling of his own suffering -- his buoyant and unflagging spirit -- was inspirational and instructive to him. "He had such grace and dignity, despite all he had been through. I learned so much from him: about myself, about life, and about class." As much heartbreak as Mark and his family endured over Sprite for such a short 26 months, they wouldn't trade it for the world.
Mark emphasizes throughout the book his conviction that as much as human beings do for their dogs they get much more in return. "There is nothing like the loyalty and love dogs have for their families. Nothing." Brushing aside the suggestion that we are God's gift to dogs, he says, "In the end, we humans are the lucky ones."