There is a theory which explains why this thing happens.
There is a man who controls dreams. He distributes right dreams to right people in right timings. The moment you fall asleep, he gives you one dream. You can neither see him nor hear his voice.
When they wake up, most people still remember their dreams. But while you feel dizzy, the man collects your dream together with your memory. The collected dreams will be distributed again. The dreams which went through a lot of people gradualy mature into dream stones.
This is only an assumption and nobody knows the truth. Nobody saw the man who controls dreams.