So here is my 2 cents on "Inside Out"
A quick review of the plot - Riley is an 11-year-old girl, who lives with her loving parents in Minnesota. She has good friends and interests, a full life for her age. We get to know the feelings inside her head, when the dominant one is Joy. Even though her family loves their home, they decide to move to San Francisco (on account of the dad's job, of course), and the physical change means an emotional one as well.
The aesthetics of the movie is incredible - the real life parts and the inside the head parts were filmed in different kinds of lenses, and you can really see the difference. A memory representation is a little crystal ball, and the most important memories, the Core memories, sit in a special place inside "Headquarters" (where we see the five emotions - Joy, Sadness, Disgust, Fear and Anger - control Riley's emotional state). Each of the Core Memories functions a "Personality Island", which makes up Riley's personality. The islands are beautifully constructed to envisage Goofiness, Hockey, Friendship, Honesty and Family. all the Core memories are happy memories.
And then the move comes, and suddenly Sadness wants to touch all the memories. Including the Core memories, and when sadness touches a memory it turns sad. And it can't be changed back. And thus comes the first sad Core memory, of Riley remembering her home and that she no longer belongs there and starts crying at school. Overwhelmed, Joy refuses to accept that a Core memory can be sad, and in the end she disposes of it, despite's Sadness's will to place it as one of the Core memories, and Joy, Sadness and all the happy Core memories get sucked into another area of the mind.
The journey back to headquarters is the Journey of growing up and letting go of the old self. You can see it in the destruction of the personality islands, in the way sadness keeps being a bigger part of Riley's emotions, of Joy's ultimate acceptance of this involvement, in the utter gut wrenching death/sacrifice of the old imaginary friend (and the funny introduction of the imaginary boyfriend - "I would die for Riley! *selfie*) and above all, the formation of the newest Core memory, which is both Sad and Joyful.
We see here how everything has to be torn down before new things build. How the move may be just a trigger for a change that was long time coming. Growing up is so many things. It's letting go of old dreams (the destruction of the Princess Dreams castle and Stuffed Animals Museum in Imagination Land, the death of the imaginary friend), it's accepting that life isn't always happy and that Sadness is sometimes necessary in order to move forward. We see the re-building of the most important things (like Family, Friendship and Hockey) and how some things are lost forever (like the Goofiness, or the utter and complete childish Honesty) and we see how perspective changes into the realization that things aren't just one thing, but many. How life can be both sad and happy, gross and scary, beautiful and terrifying at the same time.
It's an ode to the loss of innocence, of the old you, of being a child, and it's more beautiful than I can describe.
All great stories are just a mirror, and this one isn't any different. If you haven't seen it, go. Just bring tissues.