The Pixar theory

The credit of this theory goes to John Negroni, who has written the full theory.
What if I told you that all the Pixar movies are connected,  and that all the characters are in the same universe. This theory proves they do. Our story starts in old Scotland, with the movie Brave.

Brave is one of the only Pixar movies that explains the fact that animals are as intelligent as humans. In the movie, Merida (the main protagonist) discovers a magic power a witch has developed that turns humans into animals, in this case a bear. If the witch had the power to turn Merida's mother into a bear, it reasons, she would have previously experimented on other humans. During the centuries that followed Brave, the animals and objects that the witch experimented on had created a power struggle between animals, objects and humans. The struggle between humans and animals are represented in the movies Ratatouille,  Up and Finding Nemo, in that order. It starts with Ratatouille, which shows Remie (main protagonist) the rat, and his love for cooking, an activity exclusive to humans. It turns out that he is better then every human. This discovery does not remain a secret by the end of the movie, as rumors begin  to spread to others, particularly people exploring the globe, like Charles Muntz (main villain)

Hearing about a newfound intelligence in animals, he uses this to his advantage, by creating a collar that allows the animals to speak. At the end of the movie Charles Muntz dies and the dogs are free to roam the world, allowing the technology to spread.  At this point, we have seen the mistreatment of animals and them becoming smarter and smarter.  By this point, they have not done anything rebellious towards humans until Finding Nemo. This story follows a father's journey to save his son Nemo from the hands of the humans.

     We have mostly been focusing on the evolution of animals, but now we go to humans, which have evolved into supers as shown in the movie, The Incredibles. With every super hero, there is a super villain. The super villain's name is Syndrome.  He creates a machine called the Omnidroids, which are hyper intelligent superhero killing machines that can learn and adapt. By the end of the movie, we see the Omnidroids turn on Syndrome, defeating him in a matter of seconds. At the end of the movie the Omnidroids and Sybdrome are long gone, but the artificial intelligence (AI) still exists. Although it was originally developed to destroy humans, it didn't necessarily mean that all would behave this way, losing its drive to kill.
   
     Toy Story takes place about 40 to 45 years after The Incredibles.  Zero-point technology (theorized by syndrome in The Incredibles) is electromagnetic energy that exist in a vacuum. This energy which exist in wavelengths, explains why the toys can move. We first see the toys becoming fed up with their treatment by the humans in Toy Story, then we see them rebel against Sid 9main villain). In the second movie, Jesse (fem-fatal) resents her previous owner for abandoning her, then we see Lotso's (main villain) hatred of the humans for his abandonment in Toy Story 3. We never get to see a "planet of the apes" style uprising where the toys take over the humans, because toys actually need human attention to come to life. For that reason the AIs would also need human life to work, although the Machine still want control.

     Now enters BNL, the faceless company known as Buy n Large. It was created when the AI from the Incredibles went off to do it's own thing. This company is the basis for all the corporate examples in every Pixar movie. We first see this in Up when BNL tries to seize Ed's house. In almost any other movie you can find signs of the  company everywhere, from the makers of the batteries that powers Buzz Lightyear (secondary protagonist), to logos strewn throughout the movies. BNL takes over the world leaving one thing, Cars (pixar movie), which are intelligently human like, with incredibly advanced AI. Machines need humans to keep them functioning, explaining the energy crisis in Cars 2. However, since there are no humans left on earth, the cars slowly start to die. 

     Bringing us to Wall-E. In the movie humans are on a ship in space called the Axium (one of the ships supporting human life is space). We see that all along an evil AI has been controlling the ship with out the humans knowing. The end of the movie you see the humans return earth and plant a tree, which has a surprising likeness to the one in the movie, "A bugs life".  Flick (the main protagonist) is warned not to leave the island for fear of birds snakes and other bigger bugs, not humans. Which is another reason to believe that A bugs life is way in the future. Their would not be a lot of them, and so they would not be as much of a threat.

     Humans, machines and animals exist together until a newer species arrives know as monsters. Monsters Inc is a movie that takes place about 1400 years after Wall-e.  There are two theories on how these monsters came to be. The first which is that humans have evolved after living on a radioactive planet for 800+ years, or that humans have crossbred with animals to create some kind of super monster thing.  Everything needs humans to survive so it goes to reasons that the monsters would be as well, and they do. Harnessing the fear of humans as power.  You might think that humans would still be alive and thriving, because we see monsters and humans interacting. What if instead of the doors going into alternate places, they went back in time when humans were still the dominant species, harvesting greater amounts of fear. This also explains why the Mosters Inc corporation would be so paranoid with anything from the human world entering the monster world. It could cause a butterfly effect and alter the future. If you recall, Boo ( supporting child role) made it through to the monster world  and after not being able to see that world again, became her life's obsession to try and find it. So she went searching the globe for this world and on her quest she found, magic, and used this magic to travel back to old Scotland. Yes, Boo is the witch! You may say this is complete craziness, but there's actually proof to back this up. When you are in the witches hut, you can make out a wood burning of Sulley,( the main monster protagonist that boo falls in love with in Monsters Inc). as well as a pizza truck that she is carving (has an appearance in every Pixar movie).
 It would not make sense for her to know either of these things unless she had previously experienced or saw them.  As well as the fact that she uses the door to transform the contents of her hut. That is your proof! I will say that this is just a theory, coincident or not.