Ice

During the ice age, many animals died of cold, so the porcupines decided to band together to provide one another with warmth and protection. But their spines or quills kept sticking into their surrounding companions, precisely those who provided most warmth. And so they drifted apart again.
And again many of them died of cold.
They had to make choice; either risk extinction or accept their fellow porcupines' spines.
Very wisely, they decided to huddle together again. They learned to live with the minor wounds inflicted by their relatives, because the key to their survival was that shared warmth.