I bet you can think of many reasons why you should exercise, but some days, well, you just don't feel like it. Nobody is in the mood to work out every time they're supposed to, because it's hard work. It requires effort to gather up your gear and head to the gym. If you're not feeling very motivated, it's easy to be lazy on the couch and skip out on the workout.
So on days when you're not 100 percent into exercise, just go anyway. Nine times out of 10, you'll be happy and proud that you gave yourself the push. I'm not saying you need to do an intense 45-minute sprint and an hour of strength training. Be a little flexible and choose workouts that are easier on days that you have less energy. Go for a walk instead of a run. Go to a relaxing yoga class instead of cardio kickboxing. Or just do your normal workout but shorten it.
Most of the time, the biggest challenge is making the decision to work out. So don't even give yourself the opportunity to talk yourself out of it. Just go, get it done, and afterward, you know you'll be happy you did.