I know some people didnt like this movie (probably because the hype poisoned the well of their expectations and pretentions; this is a treat best served handwrapped and left on your doorstep, not jazzed up, double dipped and shoved into your mouth). To me this is a love story and like all good love stories it doesn't make the "love part" the plot but instead makes it the setup.
I've never understood romance movies where the romance is the core plot; it mostly feels cheesy and redundant. Everyone knows the best romances in movie history exist within the SIDELINES of the story. Examples: Tom Hanks and Robin Wright Penn in Forrest Gump, Han Solo and Princess Leia in Empire Strikes Back. And the movies that tell love stories the best are the ones that display them as secondary in terms of the plot. This way the audience focuses on "the plot" (a war, a bank robbery, whatever)while leaving their guard down once the romance comes into the picture thus making the romance more poignant and effective. Movies like The Professional and True Romance successfully pull this off and they are the better romance movies for it.
"Up" falls into this category and does it as well as any movie I've seen. The film also manages to prove that a story about real adult life and death issues and emotions doesnt have to have cynicism to be convincing and honest and real. I thought it was a stroke of genius to open the film with this beautiful mosaic of a richly lived romance that ends tragically in order for the story to begin. This way your feelings about the love/tragedy throughout the rest of the movie are exactly as they would be in real life after any tragedy, always lying painfully just beneath the surface. The rest of the movie is just an excuse to cover up that pain while allowing it to seep through at certain moments when its crucial towards the self-discovery of the protaganist.
I thought the movie brilliantly captured the horrors and wonders that only the really young or the really old have access to. This film is like the shedding of everything that is flesh...and yet it remains human. People argue that the plot of this movie (meaning the dogs and the villain) was weak and illogical but I think the plot was only sort of pretending to be the plot. The real plot was the old man coming to terms with the loss of his wife and the regaining of his innocence through friendship. Like a gemstone that requires a certain amount of light to see its colors this film requires its viewer to, much like the main character, hang up his cynical coat for a moment and allow for the kind of wonder and heartbreak that at their most genuine are as inseparable as they are necessary.