
From unknown times people treated dreams as most important to them. Dreams have made people to wonder, fear and enjoyment too. Priest made them sacred, when dead ones appeared in their dreams, people feared,
"That entity (the self), after enjoying himself and roaming in the dream state and merely witnessing the results of merits and demerits, hastens back in the reverse way to its former condition, the waking state." - Yajur Veda, Brihadaranyaka Upanishad IV, III-Investigation of the Three States, 34
Then, as children grow into young adults, more of the history and culture is revealed. Adults then take responsibility for passing on the stories to the following generations. In this way, the Stories of the Dreaming have been handed down over thousands of years.
http://www.dreamtime.net.au/dreaming/index.htm