Logging your workouts can be motivational because you can see your work accumulate and your efforts start to add up. If you miss one, just look back and see all the workouts you that you did do and know that you are still ahead of the game!
Also if you feel like crap or can only do 20 minutes, at least it’s in the record books! It always makes me happy when I don’t have to write a big fat ZERO in my journal. Every litle bit counts!