I find myself feeling the same toward my dissertation and ultimate defense as I did toward my black belt test several months ago. After years of training, I know exactly what I have learned, what I have accomplished; the only reason to stick around through the final strenuous test is to get an official acknowledgment of what I already know. But I can't quite bring myself to care about that official piece of paper. It'd be nice to have, but I'm not sure it's worth all the hoops I have to jump through to get it. It's just a piece of paper.
When I expressed this lack of caring to my advisor, and to a teaching advisor, the reaction was the same: You might not care about that piece of paper, but other people care! Word for word, both times - other people care. This, to me, is a ridiculous response. Sure, I care what other people think, but only to an extent. As a researcher, I care about what other researchers with expertise in my field think of the quality of my research, but only whether it is well designed, executed, and communicated, not whether it's worthwhile. As a teacher, I care about what my students think of my lessons, but only whether the lessons are effective and engaging, not whether my class is "fun" or "easy". Certainly I care about what other people think, but not enough to bend over backwards for them; not enough to subject myself to six strenuous and unpleasant months doing things I do not enjoy, sacrificing things I do enjoy, for the sake of other people's opinions.
And who are these "other people", really? A tiny, tiny percentage of the population attempts the PhD. Those who do not attempt it, those who attempt it and ultimately decide against it, they might - might- admire me for doing it, but they aren't going to think less of me for not doing something they did not do. So really, the only "other people" who care would be those who have a PhD, and think less of those who don't get one. In an administrative sense, it cuts me out of jobs that only look for the PhD itself as a marker of knowledge and skill; that cuts me out of a number of research jobs I didn't want anyway, and makes it more challenging to get a liberal arts professorship (or impossible, of "ABD considered" is only lip-service), but still leaves plenty of perfectly acceptable and interesting career opportunities open.
I do have my black belt now. So how did I motivate myself through it, and can it help with the dissertation? Well, I didn't make it through my black belt test for the prestige of having that recognized symbol of martial arts prowess. I did it as a challenge to myself, to prove to myself that I could survive the most physically demanding seven hours of my life, that I wouldn't quit when my body started cannibalizing itself for energy. Knowing what I would think of myself if I quit kept me going, except for one moment of semi-despair when the knowledge that I had carpooled to the test and would have to watch the remainder of the test kept me going.
Unfortunately, the dissertation is different than the black belt test in two critical ways. First is the time factor: putting yourself through seven hours of hell is strength, but putting yourself through three to six months of hell is masochism. Second is the criterion for success: Earning my black belt required demonstrating specific skill and convincing a single Master to promote me, but earning my dissertation requires demonstrating "a significant contribution to the literature" and convincing five academics with disparate viewpoints to promote me. I walked into my black belt test knowing that if I stuck it out and did my best I would pass; there is no such guarantee at the dissertation defense.