A long time ago, a Japanese wizard named Tsukino Mamoru (Protector of the Moon) lived. He was a great potion-master in particular, and was responsible for the discovery of many potions. His most famous potion allowed brooms to fly (all technology comes from Japan). That practice was later upgraded to a system of Charms, but that's another story entirely. Anyway, he also enjoyed looking at space, and therefore tried to concoct potions that would allow people to explore space some, but that was in vain. So, he remained content to gaze at space in his spare time, and attempt to notice things. On very rare occasions, he managed to talk to a centaur about space, and from them, learned that a-centaurs, the best stargazers around, decided the moon was very important to earth and b-centaurs were only afraid of wolves. Therefore, Tsukino-san (that's like saying Mr. Tsukino) began to idolize the centaurs, and became afraid of only wolves, and looked at the moon more and more.
But one day, he heard a rumor that centaurs though he was the most horrible thing to come to stargazing, and didn't know the first thing about the moon. Being a potion master, Tsukino-san got his revenge. He developed a potion to make certain humans become wolves (the centaurs' fear) on the full moon (their favorite time of the month). He slipped it in the dinner of a few international people, and he had faith their bites would spread throughout the world. And that is why werewolves become wolves at the full moon only.