It's true: even the most active people can get a bit unmotivated at times. And unless your profession entails a high level of activity, you have to make time for getting in some form of physical exertion. On any given day, you're going to wonder why you should even bother to move a muscle after you get home from work, especially if you spend the day stuck behind the computer - your eyes are tired, your brain is tired, sometimes even your heart is tired (emotionally, not physically).
But there's good reason why you have to do away with the excuses and get yourself moving to the point of sweating at least 30 minutes a day - and if you can't get it done daily, at least not miss more than 3 workout days consecutively. It gets your mind clear and helps you mentally organised - if you get moving at the beginning of the day, it gets you decluttered to start your day. If you get it in during lunchtime, it helps get your head ready for the remainder of your day. If you get in your workouts in the evenings after work, it pushes the frustrations and drama of the day off you and helps you boost your endorphins to celebrate the good things that have happened throughout the day as well.
Bear in mind, I know it can be a tall order to fill. But even on your droopiest days, working up a good sweat makes things feel so much better. So of the million excuses you can find not to exercise, the best after-effects - clearer mind, endorphin boost, gaining strength, getting in better shape, and who can forge the power of the after-burn (with some workouts!) - are more than enough to beat them all. So get on your gym kit and start sweating!