Why do people turn into Werewolves only on a Full Moon and never on a normal Moon?

Werewolves change any time they want not just full moon.

And they transform they don't change.

It depends on what story you are bassing on. Legends change over time,and more stories are added often, so know one know really if they change just on fullmoons, or any night, or anytime. I think that, if you go by the fullmoon therory, they transform then because wolves are known to howl more on fullmoons. Sometimes, in other therories, they transform willingly. Like whenever they needed there wolf instincts for help.