Question: How come Sadness was the only one who could change the emotion of the memory orb? Don't we sometimes look back at things we thought we were scared of or sad about, and laugh at them? How come the other emotions can't change them the way Sadness can?
Question: When Arlo and Spot first see another human, Spot moves toward that human as if he was willing to join him to bond as a family. Arlo stops Spot from approaching the other human and places him on his back so to continue taking him home. Arlo decides he wants to share his family with Spot and keep him safe in his family house. By the end of the film, the same human Arlo and Spot saw in the distance earlier shows up along with his family. Spot joins the family. Why does Arlo decide to let Spot go this time?
Chosen answer: Because he's realized that Spot wants to go and should go with his people. Before he was afraid of losing him.
Answer: Sadness can change the memories because of Riley's current situation: she has left the home she loved to move to a new city. This makes her happy memories more susceptible to being changed to sad memories. As she remembers things that used to make her happy they now make her sad because she hasn't accepted her new house as home yet. Presumably under the right circumstances the other emotions could do the same thing, just not all the time.