When you start writing, you fall in love. The characters are wonderful, the plot seems to be going well, and...
Oh. You hit a snag. Not just a little snag, either, we're talking like a tree-fell-on-the-plot snag. Something's not working...you're totally stuck. Suddenly it seems so easy to just give up and walk away from it, forget how much you enjoyed writing before you got stuck.
But you don't. You take a deep breath, get some advice from other writers, spend a while working through the snag, and press on. You keep going, keep going...until you finally finish. The feeling is...unbelievable (And believe me, I know from experience. ;) --you just wrote a whole book! :D
It can be so tempting to quit when the going gets tough. But DON'T DO IT. Whatever you do, push through it. Make drastic changes if you must, but don't just abandon your story because it's not working out right. It will only make it that much easier to do the same on the next project...and the next...and the next. Even if you think it's not that great, at least finish it. You can revise or trash it afterwards, but it's always better to finish what you start. To borrow from Nike: