1. Jon Negroni claims that the last 14 Pixar movies all exist in the same universe.
His theory was inspired by this video.
2. The timeline begins with Brave, set in the Dark Ages. It explains why animals can have human characteristics.
In the movie, Merida discovers magic and accidentally turns her mother into a bear. Other inanimate objects come to life, too.
3. Magic is provided by a witch, who mysteriously vanishes every time she goes through a door.
4. The animals affected by the witch interbreed, which explains why they can talk in other Pixar movies.
As a result, animals become smarter and gain more humanistic qualities.
5. Up’s Charles Muntz creates technology to harness the animals’ power.
But Dug, the dog, foils his plan, and the relationship between humans and animals worsens.
6. The animals’ intelligence grows, which is proved in Finding Nemo.
7. Resulting in an industrial revolution that begins in Up, run by the corporation Buy-N-Large (BNL).
8. BNL is also the reason that humans in Wall-E flee to space, proving that technology gains power over humans.
Buy-N-Large is also evident in the Toy Story series, too.
9. The Incredibles’ Syndrome uses this technology in attempts to create a genocide.
“Syndrome creates the ‘killbot’ that learns the moves of every super-human.” But this killbot turns on him, suggesting that machines and inanimate objects are trying to get rid of their biggest threat, aka humans.
This also affects the plot of Toy Story 3.
10. Technology’s rise and the industrial revolution leads to pollution.
There are NO animals in the Cars series.
12. Humans become dependent on the machines.
13. But in the end credits of Wall-E, life returns to Earth, resulting in A Bug’s Life.
14. In the meantime, a new super species is born.
“The monsters’ civilization is actually Earth in the incredibly distant future… It’s possible that the monsters are simply the personified animals mutated after the diseased earth was radiated for 800 years.”
15. The monsters have an energy crisis, which is supplied by the humans… by way of time travel.
Each door is a time machine, and it’s the only way they come into contact with humans. That’s also how the monsters harvest energy.
16. But what happened to Boo? She now knows of a future world.
“She became obsessed with finding out what happened to Sully.”
17. “She remembers that doors are the key to how she found Sully in the first place and becomes…”
18. THE WITCH FROM BRAVE.
She uses the will-of-the-wisps for time travel, which explains why she’s now in the Dark Ages. In other words, she magically creates these doors to find Sully.
19. And we know this because of the carving of Sully on her wood.
And plenty of other reasons, as explained here.