Rex and Lyra

Last weekend we dogsat for a friend of mine from school. His puppy is a yellow lab (4 or 5 months old?) named Lyra. She and Rex had a grand ol' time chasing each other through the house and fighting over toys. Finally one would get tired and finally lay down and the other one would go attack the one laying down. It didn't stop for about 24 hours. Remind me again why people have two dogs?