Why?

Why? Why does a dog want to be running around when they are on a lead, then when off lead mot want to?

This is Missey sitting in the middle of the paddock just lying there sniffing in the wind. She showed no interest in her ball on a rope or a biscuit and just sniffed. Goodness knows what she could smell, but it must have been pretty interesting to her. Later she resumed her usual biscuit loving self.