The condition of werewolf-ness, or "lycanthropy", was first described as a form of punishment for the afflicted individual's actions. There is an excellent description of the history of lycanthropy on the Wikipedia page for "werewolf".
The earliest stories about werewolves come from Late Antiquity, and typically explained the condition as a punishment from the gods for some heinous crime, such as murder, cannibalism, or impiety.
From the wiki page:
In the second century BC, the Greek geographer Pausanias relates the story of Lycaon, who was transformed into a wolf because he had ritually murdered a child. In accounts by the Bibliotheca (3.8.1) and Ovid (Metamorphoses I.219-239), Lycaon serves human flesh to Zeus, wanting to know if he is really a god. Lycaon's transformation, therefore, is punishment for a crime, considered variously as murder, cannibalism, and impiety. Ovid also relates stories of men who roamed the woods of Arcadia in the form of wolves.
And:
The curse of lycanthropy was also considered by some scholars as being a divine punishment. Werewolf literature shows many examples of God or saints allegedly cursing those who invoked their wrath with werewolfism. Such is the case of Lycaon, who was turned into a wolf by Zeus as punishment for slaughtering one of his own sons and serving his remains to the gods as a dinner. Those who were excommunicated by the Roman Catholic Church were also said to become werewolves.
And:
According to Armenian lore, there are women who, in consequence of deadly sins, are condemned to spend seven years in wolf form. In a typical account, a condemned woman is visited by a wolfskin-toting spirit, who orders her to wear the skin, which causes her to acquire frightful cravings for human flesh soon after. With her better nature overcome, the she-wolf devours each of her own children, then her relatives' children in order of relationship, and finally the children of strangers. She wanders only at night, with doors and locks springing open at her approach. When morning arrives, she reverts to human form and removes her wolfskin. The transformation is generally said to be involuntary, but there are alternate versions involving voluntary metamorphosis, where the women can transform at will.
And:
In Hungarian folklore, the werewolves used to live specially in the region of Transdanubia, and it was thought that the ability to change into a wolf was obtained in the infant age, after the suffering of abuse by the parents or by a curse. At the age of seven the boy or the girl leaves the house and goes hunting by night and can change to person or wolf whenever he wants. The curse can also be obtained when in the adulthood the person passed three times through an arch made of a Birch with the help of a wild rose's spine.
So in the earliest tales of werewolves, one became a werewolf because of some evil deeds one had performed in the past. The concept of lycanthropy being the result of a kind of "infection" transmitted by a bite is a relatively recent invention.