Sometimes it may not be clear why you're writing a dissertation.
I've had people tell me that they didn't believe in their work, that they weren't sure whether they should try to finish, whether they wanted to finish, whether it would make any difference whether they finished.
Other people have been more clear on their desire to finish, but weren't clear on something about the paper itself—weren't clear what they wanted it to say, or even that they weren't clear why they had chosen the topic and subject that they had chosen.
Such questions of central purpose are crucial in writing a dissertation and benefiting from the experience. If the dissertation is an exercise in overcoming apathy about a project that doesn't matter to you, to create opportunities that you don't want to explore, then why do it?
If you have an answer to that, of course, then you should do it—but then it would also make sense to stop wondering whether you should.
A good place to start is to take a moment—or a few hours, for that matter—to think through the whole project. If you're wondering whether you should, then think about why you think you should, and what else you would do if you didn't.
And then, once you've gone through that exercise—really seriously put your heart and head into thinking about whether you should finish—then if you decide to finish, you can stop wasting time asking yourself whether you should, and you can focus your energy on figuring out what it takes to finish.
Once you've decided to finish, then you have to find a reason for the project.
Even if you've already committed yourself to a specific project, there may be aspects of that project that you can focus on that will make it more appealing and more interesting.
One good way to get there is to find something that you believe in—some point or issue that you think is important, maybe a problem that you see in the world—and to use that as the focus of your work.