The majority of the movie itself is heartwarming, aside from the ending. The simple story of a family and their beloved, goofy dog.
Marley putting his head in Jenny's lap after she miscarries. While also a Tear Jerker, the simple act of sensitivity and love is incredibly sweet. Later, when Jenny is on strict bed rest due to complications with her second pregnancy, Marley sets up camp on the carpet next to the bed in order to keep her company.
John's description of Marley's behavior after they come home with their first-born son Patrick:
Marley, our wild, crashing bronco, was different around Patrick. He seemed to understand that this was a tiny, fragile, defenseless human being, and he moved gingerly whenever he was near him, licking his face and ears delicately.
The eulogy at the end from John about how dogs love you no matter what, and don't need big material goods to be happy. It generated a huge response from dog owners everywhere.
Speaking of response, the book mentions that after Marley died, John wrote an editorial detailing his exploits and how much he loved him. The next day, his inbox was overflowing with messages from people either commiserating with him over his loss, or challenging his claim that Marley was "the world's worst dog—because mine was".
The ending of the book. Despite John and Jenny acknowledging how easier life was without a hyperactive and destructive dog in their home, they didn't quite feel like their family was "whole". Eventually they come across an add for a Labrador retriever puppy for sale that looks just like Marley did, and they decide to go and see it.