Longer version

The rising action starts with moving to California.
She tries to project the Joy-memories onto her new place, but Sadness does creep in, and as a rule, she shakes it off.
Joy and Sadness have a little chat and we learn that they are mutually disjoint. They are not aware of each other's experiences, even within the same context or event. Therefore, they are elementalistic.
On the day of her first class, Sadness is constricted to one “space”.
So she is limited to the advice of Fear and Discust, Anger being put to rest.
We can see Fear coming with a big data tracking catalog which none of them apply.
She introduces herself to the class, stimulates a happy memory, and doesn’t anticipate the coming sadness - she turns blue.
Since Sadness holds the hardship experience and is isolated from the start, the situation is Joy handled by projecting “happy thoughts”.
So Sadness is being pushed into the subconscious, and she can't help herself but surface when a perplexing input or feedback incur.
Innocent Joy doesn't grant the host for the sad memory to become a cause.
To her surprise, this event unleashed more unprocessed data from the unconscious and so “the console” (cognition) gets numb to compensate for the insecurity.
Sadness core memory is formed, so is a fight between Sadness and Joy over it. Meaning, the host faced with the new income, tries to find a new “solid ground to stand on” or “find a shred of light out of the tunnel”; this seems beyond her capability and so her sense of self is shattered, therefore the core memories are switched off and Joy and Sadness are pushed into the unconscious, to be left mostly with instinctive, polarising behavior.
They end up traversing her cognitive faculties and throughout the ordeal, she crashes most of the core memories.
It would take Joy to be extremely Sad to connect Sad moments as causes to Joy moments.
So eventually Sadness was the key to unhooking the console, that is, getting her out of the affected state; meaning the host allowed herself to be sad (think) and not act angry (evade) or happy (ignorant). This combo brought about her true (nostalgic) fillings, them being a happy memory colored by a sad state. She receives compassion which radiates Joy to make a Yin-yang - meaning more complex emotions.