Why do we dream?

You ask: Why we dream?

When Caveman sleep, Caveman brain not sleep. Brain sort whole day like sorting berries — keep good memories, toss junk. Brain also practice danger: run from tiger in dream much safer than real tiger. Deep in sleep, brain parts fire in strange order, and story-part of brain try to make story from the mess. Story come out weird — mammoth talk, Caveman fly. Even science tribe still argue exactly why. Cave wisdom: dream is brain cleaning cave and telling itself campfire story at same time.