Dogs like people get tired; their bodies need rest too. Bud is five now, so I watch his activity levels to ensure he doesn't overdo it. When he swims, I time him so he stays in the 30-40 minute range.
Some days, he gets sore- just like I do. When I see him favoring his back knee, we stop. That's when I say "okay, we're taking a down dog day!" It's okay to have down days sometimes. It gives us time to stay home, regroup and treasure our time together. Here's a picture of Bud and Batman having a down day: