Artemis, goddess of the wilderness, takes virginity seriously. Like really seriously. A sworn virgin herself, the huntress goddess also makes all of her nymph followers take vows of chastity. So when Callisto's pregnancy is revealed, her patron goddess totally flips out. Even when Callisto tries to explain that Zeus seduced her in the form of the Artemis herself, the goddess doesn't have any pity for her.
In some versions of the story the goddess's punishment is even harsher than just turning Callisto into a bear; on top of that, she also shoots the nymph with an arrow. (Yikes.) No doubt about it—in the story of Callisto, we definitely see a harsher side of this nature goddess.
For the softer side, check out Shmoop's guide to Artemis.