I ran into a similar issue a few months ago. In my case I was just tired of working out and eating healthy for no particular reason. I was always asking myself why I was spending so much time lifting, doing cardio and preparing healthy meals....other than to be healthy of course, but it wasn't enough for me.
What I did was set a goal. I needed to accomplish something I hadn't done before. Something to push myself and give me that motivation that I was lacking. I decided to enter myself into a men's physique bodybuilding competition. Instead of lifting and dieting for size and muscularity, I am working for the magazine cover look in boardshorts on a stage in 14 weeks. I've never done anything like it before and it's an interesting experience.
I found a fitness consultant to tell me what to eat each day and which lifts to do to supplement my physique and it has really pushed me!
I probably didn't need to go though all that, but I hope you get the idea.
Set yourself a goal for something that you haven't accomplished and maybe would like to say that you did down the road. Maybe it's a marathon, a competition of some sort, or an intramural sport. It's up to you. You get a new experience, new friends, and that sometimes difficult to grasp motivation.