Angst Aversion: Just look at the Tear Jerker page! People have given countless stories about how the ending, in which Marley is slowly succumbing to his death from old age, was impossible to handle for them; some even threw away their books at that specific part or left the movie theater in complete tears, even when the movie hadn't finished yet.
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.)
Base-Breaking Character: John and Jenny themselves. Debate on how sympathetic they are of their frustration towards Marley still rage on. This is less so with the movie version because of Adaptational Heroism.
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.
It Was His Sled: Marley gets put down in the end. Ever since the movie came out, thanks to its pop culture status as "default sad movie", the words "dog dies" are the association words for anyone who hears this work's title, whether they've seen it or not.
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.
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.
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.)