Since the invention of agriculture, mankind has been obsessed by the seasons, which form a life and death cycle for vegetation on Earth. Since human being believed they had a role to play to keep this cycle going and ensure the next harvest, they often sacrificed to the gods. In all early civilizations, this also meant the sacrifice of human beings.
This sacrifice was sometimes done willingly. It was an honor to contribute to the cycle of the universe. Evidence for these types of burials is also found in early literature. In some versions of the Epic of Gilgamesh, we read of the funeral of king Gilgamesh. During the ritual, his wife, members of his family and members of his court, willingly go down into his tomb to accompany him in the next world.
"The Great World History Book"