Such a loss would devastate any child. With such a loss, any parent would understand a child's despair. Instead, Riley's mother pressures her into hiding her sadness and being happy.
Thus begins the struggle between Joy and Sadness, two of Riley's five emotions, but something goes terribly wrong when both of them fall into Riley's subconscious leaving Anger, Fear, and Disgust in charge.
Without Joy, Sadness, and all five of Riley's core memories, Riley loses her personalities. All she can do is express her remaining emotions, and it's not pretty. She gets herself into trouble, even tries to run away, a decision that nearly plunges her into complete darkness.
There only way to save Riley from becoming completely catatonic. No only must Joy accept Sadness as her equal, but she must allow Sadness to drive Riley through her challenging times. Joy must relinquish control.
Without sadness there cannot be joy. Without joy there cannot be sadness. Unlike the other three, sadness and joy need each other, which is why they are both lost together. They are both recovered together. They even share the same eye and hair color, much like sisters.
In the end, all five emotions find a way to work together instead of fighting for control, and Riley is able to progress to a new level of maturity because of it.
Pixar does a fabulous job in the creation of this film. I could literally feel every emotion that Riley feels: joy, fear, anger, disgust, and most of all, sadness. I can't watch the end without crying. It is a colorful roller coaster from start to finish.