Because you will have done the correct amount of exercise and conditioning to get it that way. No animal alive can be fit for hard work unless it has been conditioned. Working dogs are the equivalent of human athletes: they need to train hard to achieve the right level of fitness or they'll damage muscles, lungs, heart, toes etc if they are run unfit.
Dogs should have a minimum of an hour's exercise every day in my opinion, and part of that exercise should be steady trotting, not slow walking, but trotting. A few good gallops in between and the dog is half way to fitness. People who take