A very long time ago none of the trees lost their leaves. The leaves would die and drain the trees life force and soon the tree would die too. Evergreens had a different kind of leaves that didn't die in the cold, so the God and the Goddess asked the evergreens if they would stay awake all winter so the maples, ash and oaks could drop their leaves and sleep during the winter. Then in the spring the leaves would regrow and the trees would wake back up, refreshed and healthy. The evergreens agreed and that is why some trees lose their leaves and some trees don't.
He asks for this story a lot and has even told it to his class. I thought it was kind of cute.