Following on the thoughts of the last two days, there's still a little more to add (but only a little).
The sense of purpose starts in us. Whether we're talking about the meaning of a sentence, the meaning of a paragraph, the meaning of a paper (or dissertation) or the meaning of our life, it starts in us.
It starts with things that we care about, with things that move us emotionally.
Viktor Frankl's "Man's Search for Meaning" develops a school of psychotherapy based on the idea that if we find meaning in our life, then we will be able to maintain a positive emotional state.
If we look outside ourselves for that meaning, then we are at the mercy of outside forces. If we find it in ourselves, then we're able to find stability even when outside situations change.
For obvious reasons, this is crucial in writing a dissertation and in life, in general. If we write our dissertation on the premise that we will find meaning by finishing it, or because we hope to please our readers, there's little positive emotional force behind it. If, on the other hand, we write it because we care about what we are writing, the force drives us.
When writing my dissertation, I felt despair when I focused on the hurdles to pleasing others and the slim hopes of getting published. When I focused on my belief that I had something worth saying for its own sake, I was able to push forward. time and again with my clients, I have noted how their ability to work changes as they bring themselves back into touch with a sense of purpose for their work and a sense of having something to say that is not just an academic exercise, but that touches on the ideas and issues they find most important in life.
So, to wrap up, find your sense of purpose--for your sentences, for your paragraphs, for your dissertation and your life. Don't let others reduce these things to interpersonal games; hold tightly to your sense that you're doing something you care about--that way you won't feel like you're spending your time becoming emotionally impoverished.
Hopefully this isn't too repetitive of what I've written in the last two days.