Why did Tāne marry his own daughter?
In Maori mythology, the progenitor of humanity was Tāne, a child of Father Sky and Mother Earth. In the more popular version of the tale, Tāne - desiring a mate - created the first woman using the soil of Kurawaka. Her name is Hineahuone, and together they had a daughter, Hinetītama.
However, Tāne married Hinetītama without the latter knowing his identity. When she finally found out, she fled in disgust to the underworld, becoming the goddess of death, Hinenuitepō. There she waits for her children to join her in after life. This is said to be the start of death for humans.
But why would Tāne marry his own daughter? How is it that Hinetītama did not know she was marrying her own father? Was her mother Hineahuone not around to tell her?
Excerpt from Hinetītama's origin story:
Tanematua took her as a wife and in the Aonui month of the Orongonui season (Pipiri) she gave birth to Hinerauwharangi. After a while Hinetitama, watching her child with her husband, became curious as to who her father was, for she had no recollection of her father. She pondered on this for a few months and then asked Tanematua. He evasively referred her to the posts of her mother's house.
A great dread came over Hinetitama as she began to suspect the truth and asked a second time. Tanematua did not reply, but made an unmistakable gesture. Hinetitama, so shocked, told Tanematua that she could not continue in the world of light but would seek the protection of her grandmother, Papatuanuku and would retire to the lower world.
Her reply epitomised her grief and abandonment, "The path of Tahekeroa to the lower world shall be layed down for all time. From the Muriwaihou I will look up to you and our offspring moving in the world."
I think it's just who he was. Here is an excerpt of his description in A Concise Encyclopedia of Maori Myth and Legend:
Since the word tāne ordinarily means 'male, husband, lover,' Tāne's name is a personification, Male. In the myths relating to him, male energy is presented as having shaped the world and created the life forms that belong to the land. Every human man - every tāne - who fathered a child was re-enacting the occasion on which Tāne, having obtained a wife, fathered the first of his children.
He married women because that was the essence of who he was. This is probably also the legend that taught the Maori not to commit incest - in order to teach a lesson, you need to have a story that says why it's a bad idea; marry your daughter, and she will become the Goddess of the Underworld!