Have you ever skipped a workout?
This question reminds me of the adage that the only way to never fall off a horse is to not ride. If you work out, sooner or later you're going to want to skip because you just don't feel like doing it. If it happens too often, your fitness plans (or dreams) are doomed. And let's face it, that's pretty common, and is partly why every year the gym crowds of January dwindle back to "normal" by March.
But it doesn't have to be that way. While never missing a workout would be ideal, there's a compromise: on the days you feel like skipping, feel like doing nothing, just do something. Anything.