Over 10 years of training i've had these regularly. I have found it is one of two things:
Of course, the main thing you look at is your motivation, you feel as though you're lacking and it is a mindset thing and you push yourself through, even though you're not feeling it.
The second (which can be misconstrued for the first) is actually a symptom of over training. If you've been training for a while without a break (and I mean a true break) take a week or two off the gym. You will be surprised how energized you are.
Note; taking a week off is for those of us who train incredibly hard. I've had clients who feel the need to take a week off here and there when they've missed weeks, or only trained once that week. If you've been hitting the gym hard 3-4-5 days a week without a week off for months on end - it is often a good idea to take one off. It will lift your mood, spirit and energy levels. Worked for myself, anyhow.
Shrek: you leaned out? Looking good mate!