In the mornings, just the peacefulness of it all is what motivates me. Not many cars on the road, birds chirping, the sights around town are just very peaceful. On the ride home, its the last two miles when i'm on my MUP just whizzing by all the cars waiting to get through the intersection and knowing that I'll get through the light before many of them.
Honestly, this being my second year of riding to work what I find is just the peacefulness I get riding, taking my time and getting to notice more of the sights along my route that I would miss if I was in the car. I just find it more enjoyable the more I do it.