Years ago, my dog, Louie, and I used to love going on a run together. For the first few years, he was hard to keep up with and seemed like he could go forever, always looking back saying, "Come on, dude, let's go!" As he got a little older, he slowed down a bit. At first, I thought he just felt bad for me and was being merciful, but then I realized he was getting tired and he couldn't run like he used to. Now, we shifted to short walks as running isn't in the cards anymore. He lost the capability.