Why are Rest days so Hard?

I find when I train my REST days are actually one of my toughest days. I haven't truly figured out why 'resting' seems so difficult. Maybe it is because I am in such a groove and then all of a sudden I am supposed to REST. Or maybe it is the fear of enjoying the rest day so much that I won't want to get up the next day to continue my training. I have read so much research that supports 1-2 rest days a week, totally taking the whole day off! Research claims rest helps our muscles repair themselves, allowing us to take full advantage of training. And, of course, REST allows us time to mentally focus on our other training days. So it all makes sense, but why is it so hard? Seriously, the more you train, the harder REST days are. Once you've got a momentum going in your training, there is fear of resting.....will it impact your next day's training regimine? Will you loose your drive? I continue to wrestle with REST Days however for fear of overtraining and injury, I take the rest days. And, we all know that REST Days don't always allow us a day at the spa (wouldn't that be nice??); my rest days involve 3 very active children. So, it might not be my day to sit and get a pedicure but it will be a day I take it easy...no matter how hard it is!