You see, dogs are known to chase small, round objects known as tennis balls. 

I'm sure the ancient Scandinavians had similar balls that they used to play fetch with their dogs.

It also just so happens that the Sun and Moon are round...like a tennis ball.

All along, the tale of the two wolves chasing the Sun and Moon through the sky probably has its origin as something humorous to tell children; the wolves are simply playing fetch with the celestial lights.