One of the questions that often come up when I talk to my friends about work and career is, "Why keep going?" There clearly are a million reasons why people keep doing what they're doing, but, because of the way my brain works, I can see four categories for these reasons which go from good to bad.
I don't think that there can only be one reason or one category; we are complex people with complex emotions, but I've found it helpful to try and better understand them so as to make better decisions.
For obvious reasons, Excitement is a great reason to keep going. If you're excited about the potential, the challenges, the unfinished business, then you need to keep going. Excitement gives you energy, feeds your passion, and will sustain you when you have little else.
Loyalty can be a good reason, and I've seen it manifest as loyalty to a person, a team, a memory, a promise, a cause, a project, a company, a brand. It may also be bad when it's at the expense of one's own happiness or when it's loyalty to something that isn't deserving of it.
Inertia is a powerful force, and it's easy to let it take over. It's easier to wake up every day and go to the same place to work with the same people on the same problems and then bitch about it to the same people, instead of forcing oneself to take a chance and do something different. If Inertia is the primary reason to keep going, it's definitely worth reconsidering.
Lastly, Fear. Fear of failure, fear of change, fear of loss. With any change there is fear, and often it's healthy to have a bit of it; some believe that new challenges that don't scare you aren't big enough, but I would consider that more excitement than fear. However if fear is the overwhelming reason to stay... I believe it's a sign that you must go.