I think it's a combination of the work and the environment, but I'm not entirely sure. All I know is I don't want to do this as a job anymore, probably not even as a hobby.
In my case I know that in my case, it's the combination + the people there. I am tired to that work, I can no longer evolve there, and I do not like the most of the people work there, everyone saying "it's not my work".