For me, its fatigue. If I have had a long, hard day, or I am tired from little to no sleep the night before, I wont have the energy to initiate. If she initiates and does a lot of the work, I can be up for it. But if I am tired, I am out.
Another one that takes the wind out of my sails is food. If I just ate a big dinner or something, my drive wains considerably. Also, being sick. Even a cold keeps me from wanting to mouth-breathe through sex.