Serpents were the symbol of the life, because it look like phallus. Serpents catch rats, which means they protect people's food from rats. Serpents may kill people, which was thought as god's act.
Serpents shed their skin, which was thought as the rebirth.
Serpents and lizards, Akane Shobo Serpents were very close to life and death in the world. It would've been very natural for ancient people to worship them.