Theirs without a doubt.  They have much more energy than I do.

Hi Y'all, 

I walk my Humans for their health, of course....but if you ask them, they walk to keep me fit!  Bottom line, it works both ways...we help each other. 

Y'all come by now,

Hawk aka BrownDog

For her health, and her sanity.  My pup is aging and she needs the exercise to keep her muscle tone up.  She also needs the stimulation.  If she doesn't get to go outside and sniff stuff every day, she is a nervous wreck!

Both. I got a dog because I was, somewhat unintentionally, raised with the idea that it's weird to go for a walk without taking a dog along. (We had a border collie in an inner suburb. No one was allowed to go anywhere that dogs weren't banned without taking the dog along for some exercise and mental stimulation.)

Even when the only reason I drag myself off the couch and out the door is because the dog needs it, I wind up feeling better, healthier, and happier by the time we're halfway through our walk.