"Inside Out" tackles the concept of emotions in a way no animated film has before, and, in doing so, it forces us to examine all of the disparate parts of ourselves, even those we may have ignored for quite some time. It does this specifically with the character of "Bing Bong," Riley's long-lost imaginary friend. Even if you didn't have one growing up, the nostalgia and heartache that somehow sneak up on you are overwhelming.
The pink elephant represents the adventures that are yet to come. And so, when Bing Bong says to Joy, "take her to the moon for me, okay?" it's about ensuring that Riley will still have an opportunity to dream and enjoy life even if Bing Bong isn't there to see it. The saddest part of this quote is that symbolically it points out how life forces people to forget the glimmers of joy amidst the sadness. His final words are a way for Bing Bong to know that Joy won't allow Riley to stop using her imagination. Plus, it shows that Bing Bong never stops caring for Riley, even when she forgets him as she grows older, which makes the quote that much more poignant.
