The people in that world

all had a single box.

The thing inside the box took a different form for each person:

Their secrets, personality, precious things, ways of thinking, personal interests...

or things they liked, or didn't like...

You might call them

"the contents of that person".

Because of that, everyone

made sure to never show them to anyone.

Only the person themselves would know the contents of the box.

There was no small amount of interest towards other people's boxes, but it's not like they'd ever show them...

That was common sense among those who'd always had boxes.