For the same reason I get annoyed when I hear the praises of work/life balance. Balance to what? Leisure? Where does leisure lead? If my work can give me joy why should I strive to minimize its presence in my life (or even balance). Are there things in my work that aren't "pleasant"? Naturally - but it's part of the joy. Only through working through those unpleasant experiences do I value the end-result of my work seriously.
Granted - there are work situations that do not deliver joy. The question is why, and why is nothing done about it.
Anyways. I am of to spend my lunchbreak doing interval training in minus 2C until I am surely about to throw up on the track. Why? Because it'll feel amazing afterwards and I have that sense of achievement locking in for today. Makes me sleep like a baby.