It's the lack of feeling you've completed anything. I work for a major telecom company doing field support, mostly patching holes in the dike, and because of this, I can only rarely point to something and say "I did that" and feel good about completion or a job well done. It comes down to fighting the fire and moving on, which when combined with no feedback from management is a great way to leave work dissatisfied every day, and dreading the next.