I felt that hand in hand with the theme of the acceptance of emotions has to do with how we are shaped by our emotional experiences and how those are translated into self-fulfilling memories and therefore reactions. Riley's memories are obviously affected when touched by sadness and (now simplifying the whole Joy/Sadness rollercoaster into:) because of this Riley can no longer draw on these memories and experiences in the same way, destabilising the core aspects of her life (the islands). I realise that the logical continuation of the collapse of the islands is that Riley is bereft of personality (a little problematic a statement) but it is it is a clear trajectory into her becoming a significantly different individual, or at least one who acts in a significantly different manner. Additionally, it is the resources that Riley has accumulated through her life that see her through her problems in the real world, like Bing Bong. Similar to what you've said, Scott, I think that Joy and Sadness's understanding of each other's perspective shows a consolidation of Riley's own understanding of her experiences, eg. a healthy maturation process. So, in hindsight, less separate thematic territory and more an angle built into the same theme. It's also interesting to note on a more meta level that these "formative experiences" are the way that many writers approach creating characters - the 'what was their childhood wound' approach.