Inside Out is a coming of age story—not just for Riley, who's distancing herself from the days of princess castles and song-powered rocket wagons, but also for Joy, who learns that it's okay for Riley not to be happy all the time.

The progression of both characters is given a swift kick in the emotional pants when Bing Bong sacrifices himself so Joy can escape the Memory Dump and return to Headquarters with Sadness to restore normalcy to Riley's brain.

For Riley, Bing Bong's sacrifice symbolizes the end of childhood. Simply put, she's way too old for imaginary friends. She's moved on to imaginary boyfriends—although she'll outgrow that, too. Hopefully. We'd hate to see an Inside Out sequel where Riley, age 38, shows up at her 20-year high school reunion apologizing for her imaginary husband's absence because he's on a business trip in Antarctica.

For Joy, Bing Bong's sacrifice represents her first taste of loss, as well as her first experience with complex emotions. She's stoked she made it out of the Memory Dump and can go restore order to Riley's emotional world, but she's also gutted to see Bing Bong fade away. Looking back at Bing Bong below, she's experiencing firsthand what she saw Riley experiencing in the rewound hockey memory: a combination of sadness and joy.

Bing Bong sacrifices himself for something bigger. That ability to behave unselfishly itself is symbolic of adulthood. When's the last time you heard a kid say, "No, no, Mom. Why don't you go downstairs and rent that new Liam Neeson movie you've been dying to see? I can clean up the creamed corn I wiped on the dog and finish this potato clock myself. Then I'll tuck myself in. You do you for once."

Never? Yeah, same here.

Bing Bong's act of selflessness symbolizes a turning point for Joy and, by extension, Riley. The bad news is, life's only going to get more difficult from this point on, as they experience increasingly sophisticated emotions. The good news is, it's only going to get richer and more meaningful, too.