We want to be working from a place where we care about our work. Writing is a far cry from digging ditches. If we have let our experience of the dissertation become interpreted primarily as meaningless pain, that's no good. It doesn't help us get our work done, either.
This emotional involvement comes with it some difficulties, however. We are going to have ups and downs. Some days the work will go well and we'll feel good. Other days we will be facing difficulties and maybe feeling like quitting.
We want to be able to find a place of balance, where we are not caught in a whip-sawing of emotion. That in itself is exhausting. It's tiring enough to face the frustration of the project, and to have doubts about our ability to finish. If we have to experience severe ups and downs it may be even worse. Having our elation stripped away time after time may be more tiring than just having to face a constant battle.
If we can find a place of balance, we will not be so prone to the swings of emotion, and in particular we may be able to keep ourselves from going into the highly unproductive down swings.
I've been working with a writer who wants to quit every other day. And on the days that quitting isn't the plan it's enthusiastic work: go! go! go! I find it exhausting. Every time I try to respond to the question of whether or not to quit (a question I can't answer for the writer), What I get back is "I'm not quitting; I'm working again and I want help with _____." And every time I try to give help with _____, I get back "I'm quitting." The only way I can keep working with the writer is to keep reminding myself that the ups and downs are emotional and that I have the ability to retain my emotional balance even while those around me are losing their heads. But my difficulty is surely less than the writer's: this is a situation that is extremely difficult for the writer--the vacillation and the emotional swings are both cause and symptom of the difficulty this writer faces.
As a writer you have to keep your head about your work. You have to be able to see its strengths and weaknesses. The same piece of work will often garner praise from one corner and derision from another. It behooves the writer to let both those ups and downs pass in order to keep an eye on the balance of the work.