Why doesn’t everyone share the burden of the bad memories?

From the text:

“But why can't everyone have the memories? I think it would seem a little easier if the memories were shared. You and I wouldn't have to bear so much by ourselves, if everybody took a part."

The Giver sighed. "You're right," he said. "But then everyone would be burdened and pained. They don't want that. And that's the real reason The Receiver is so vital to them, and so honored. They selected me - and you - to lift that burden from themselves.”