It's also a bit thankless, being good as this job, in a lot of cases, means you are moving really fast from one task to the next without any down time in between. I find it hard to get to a place where I can feel accomplished or proud of a piece of work, because the next fire/high-priority task is already "behind schedule".
I think part of it may also be due to the fact that the job is constant problem solving. Even though its nice to find solutions to problems, knowing you'll be perpetually in a state of "there is a problem that must be fixed" is taxing on mental health (for me at least).