Because he wanted to be.
tefnut was the Egyptian goddess of moisture
I don't know if this answers your question how you would like it to be answered but the goddesses of moisture is Tefnut
Shu (God of the Air and Sky) and Tefnut (Goddess of Moisture).
Tefnut is a goddess, the mother of Nut and Geb, sister and consort of Shu; she was goddess of moisture and warm air.
Tefnut is the Egyptian goddess of rain dew and moisture
Tefnut is moisture, warm air.
She is the goddess of moisture.
Tefnut, the Egyptian goddess of moisture in the sky, did not have a mother. She was said to be created by the god Atum while he was masturbating. Later on, after Atum was associated with the god Re, Re became her father. source: http://news.egyptianmyths.net/tefnut.htm
In Ancient Egyptian religion, Tefnut, transliterated tfnt (tefenet) is a goddess of moisture, moist air, dew and rain.[1] She is the sister and consort of the air god Shu and the mother of Geb and Nut.-- from Wikipedia.org
Tefnut is a goddess in ancient Egyptian mythology, associated with moisture and rain. She is typically depicted as female and is one of the primordial deities, often portrayed alongside her brother Shu, the god of air. Tefnut is an important figure in the creation myth of ancient Egypt, symbolizing the nurturing aspects of nature.
Shu and Tefnut
As one of the cosmic deities of the Ennead, Tefnut was the divine personification of moisture. Tefnut could be regarded as "Eye of Re', and as such was represented with a lioness head (and was worshipped in Leontopolis).