I think a writer has to really love what she's working on in order to keep going. That might sound a bit ego-centric, but if you don't like the story - and you're the one creating it - who will?
The moment I know I love what I'm writing is when I start to read it out loud. Sometimes it happens at the very, very start and this euphoria can keep me going for the first 10,000 words. But mostly, it's later, when I look back on it, I'll realize that I like - or maybe even love - what I've done. I'll want to hear the cadence and rhythm of the words that make up my story. I will want to spend some of my precious writing time reading favorite parts out loud, just enjoying what I've created and how it sounds.
The other good thing about reading aloud is that it's a super way to edit. As you read, the parts that don't flow, that seem to "stick" will be very apparent.