What are some of the most common reasons for being sluggish and tired most days - while otherwise feeling just fine.?

Many possibilities. The way to figure out what causes intermittent fatigue is to keep a diary for 3-4 weeks and show it to the primary care doctor. The diary should include things such as medicines taken, vitamins or supplements taken, caffeine / alcohol / illegal drugs used, bedtimes, wake-up times, stressors (job / home / spouse / kids / etc.), exercising, foods eaten, meal times, etc... Blood tests might be done.
Fatigue. Many reasons to explain fatigue that could be sorted our iwork a thorough evaluation by your family physician.