This question demands a lot of detail. I'm not sure I can provide all of it. I'll do my best to sketch it in here. I'll doubtless miss something. I'll probably also get stuff wrong. There are a lot of moving parts to a Pixar movie production. There are people whose jobs involve keeping track of those parts. I never worked one of those jobs.
Sometimes parents are the ones that teach kids to squash their emotions. The parents don't want to see the fear, the pain, or the uncertainty that is clamoring for a voice. So they keep urging the children to "just be good" and "just smile" and "go along with things" and "don't make trouble" until...
In Inside Out, 11-year-old Riley moves from Minnesota to San Francisco, giving up her home, her friends, and her ice hockey team in the process. When the moving truck gets lost, she winds up in a sleeping bag on her floor. But the tragedy isn't what happens around her, but within her; not only does Riley stop feeling joy, she also becomes numb to her sadness.
You could say that mindfulness is like a personal app that allows you to see what's playing out in your mind whenever you pause to notice it. Without this awareness, the unnoticed thoughts, beliefs, memories, and feelings sit at the control panel of your mind, pushing your buttons and controlling your behaviors.