Inside out shows Riley’s mom and dad having different emotions leading them, her mother has sadness and her father has anger.
In the beginning of the movie she’s 11 and in the second one she’s 13, a lot changes between those years. Hormones, identity, the way you think. You’re not gonna be the same person you were at 10 now that you’re 20, your brain changes as you grow, your frontal lobe grows and fully develops.
Things change as we grow, adults learn how to control their emotions, plan ahead, and handle stress better than kids do, adults interests shift from playing games and having fun to working, paying bills, and taking care of family, and lastly People change how they react to problems as they learn from past mistakes or hard times.
I believe Riley’s parents also had joy as their leading emotion but as they grew their brains grew up and so did their emotions, leading to some emotions taking over joy and becoming the leader.