Trivia: Riley is the only character in the movie with both male and female emotions.
Trivia: John Ratzenberger has been in every Pixar movie to date and here plays the role of the character Fritz, who is is a greenish Mind Worker with a mustache and no hair who installs an upgraded console in Headquarters when Riley turns 12.
Trivia: When Joy and sadness accidentally cause Riley to have a nightmare the music that plays when it starts is the intro to Grim Grinning Ghosts - the theme to the classic Disney World attraction The Haunted Mansion.
Trivia: In Riley's classroom, there is a map with pins marked on different places all over the world at the back of the room. Each pin represents a location where another Pixar movie is set.
Trivia: Colette Tatou from Ratatouille appears on the copy of a magazine 'Haute Dish'. (00:06:05)
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.