No Mom
The wolf didn’t have a mother, which had probably been killed by a farmer. Therefore, the cub would die if it wasn’t protected. Motherly intuition brought his cow to the cub, and she decided to care for it.
A Happy Ending
After losing a calf, the cow ran away. It probably saw the wolf cub as a new chance to nurse a child. Since the wolf was old enough, the animal control employees could catch it and release it into the wild. Joe got his cow back, and everyone was happy.