I think writing is like falling in love with the guy (or girl) you want to spend the rest of your life with. When you first get together, it is exciting. All you can think about is them. You don't see their flaws, only the perfection in them. Then time goes by. You get to know them better. Things you used to find endearing start to irritate the crap out of you. All of a sudden, you are wondering why you chose this person. You start to doubt they are the right fit for you. At the point a lot of people give up. Writers move onto greener pastures (another story). The problem is this will happen again and again. Just like a good marriage, you have to work to create a good book. You have to stick with it even when you no longer feel that wonderful love you started out feeling, even when the excitement has died down and you don't have butterflies anymore.
Stick with it. Keep writing. Look for those things in your characters and story that first drew you to it. In no time at all, you will be in love again.
I can't even count how many times I fell out of love with my newest book: BAIT. There were times when I wanted to throw it in the trash and do something else. It seemed hopeless. Surely a good story can't come out of all this disappointment and doubt.
But it did. When I went back and read the entire finished manuscript, I was pleasantly surprised. It's official. I love my book again, and this time it's for keeps. :)