Angst Aversion: Just look at the Tearjerker page! People have been constantly giving stories about how the ending, in which Marley is slowly succumbing to his death from old age, is nearly impossible to handle; with some even throwing away their books at the specific part or viewers leaving the movie theater in complete tears, even when the movie isn't finished yet. Bottom line, this is clearly one of the biggest Tearjerker in film/literature history, period.
Awesome Music: "It All Runs Together" playing at the very end when Marley is put down. Marley's death aside, any pet owner who experienced putting their dog down likely shed a tear listening to this track thinking of their own dog (or another beloved animal for that matter.)
Hilarious in Hindsight: Reading the part with Marley struggling to behave for his brief film role in the 1996 film The Last Home Run and John enjoying their time in the spotlight becomes this when both Marley's book and film gained considerably more fame than The Last Home Run did.
A mixture of this and heartwarming: when the film was in production, John decided to keep Woodson, one of the Labrador retriever puppies that was used to play puppy Marley.
Narm: A lot of Owen Wilson's acting can come off as this. As one critic put it:
Brace yourselves, everyone, for the emotional final scene, when Marley faces the final curtain in the vet's treatment room, with blond, bland Owen at his side. And in the cinema, all of us tough, cynical critics had tears welling in our eyes, swallowing hard; our lips, so often curled in a cheap sneer, were now trembling, because of the same desperately sad thought: "Owen Wilson used to be really good ..."
Tear Jerker: Especially if you've had dogs. Doubly so if you had to put it down or it died in some way.
Values Dissonance: Some of the Grogan's more abusive behavior tends to make the audience perceive them as less sympathetic in modern eyes, as opposed to the 90s period that the book set in.
What Do You Mean, It's Not for Kids?: According to Grogan, he got a lot of dirty looks from parents who bought the book for their kids expecting a happy dog story. He eventually allowed child-friendly versions focusing more on the doggy antics.
The Woobie: Marley becomes one in his later years.
They Changed It, Now It Sucks!: Was obviously trying to cash in on the Air Buddies movies considering how much more childish and slapstick it is. Lacking any of the charm and heart of the previous entry.