What is the theme in Marley and Me by John Grogan?

Love and devotion is a major theme in the story. Dogs have an amazing ability to exhibit an intense love and devotion for their families that is not mimicked by other animals - humans included. There is a pureness to the loyalty shown by dogs that humans are incapable of mirroring, due to their ability to think and judge. Dogs are happy to give their owners unconditional love and ask little in return except food and some play time each day.