The reason they all have good stories is because they aren't at all original (at least the ones I've seen, which certainly isn't the entire Pixar catalog). They are essentially all new twists on centuries-old stories that have proven to be popular and have stood the test of time. I could probably think of a dozen movies/books/stories just off the top of my head where a child is separated from his guardian, go through trials and travails, enlists the help of kind strangers, has some adventures, learns some life lessons, then eventually gets ruinited with whoever he was separated from in the first place. Joseph Campbell could probably name a thousand. That's not to say it wasn't good. It was a new take on Campbell's monomyth, pretty much a step-by-step re-write of the Hero's Journey.